docs: document per-signal calibration and config save/reload

Update Docs/StreamHub-API.md (new setCalibration/reloadConfig commands,
calibration/configSaved/configReloaded events, §4 config file format),
ARCHITECTURE.md §6 (updated command/event tables and Config File Format
subsection), Docs/WebUI.md (Cal row in V-Scale Toolbar, Sources & Config
sidebar section). Also corrects the spec's Validation sentence to match
the shipped cal-invalid border behaviour instead of silent field revert.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ARCHITECTURE.md

@@ -380,7 +380,9 @@ binary frames carry data push payloads.
 | `ping`           | — | Hub replies `{"type":"pong"}` |
 | `addSource`      | `label`, `addr` (`"host:port"`), `multicastGroup?`, `dataPort?` | Connect to a new UDPS source; hub assigns id `s1, s2, …` |
 | `removeSource`   | `id` | Disconnect and remove a source |
-| `saveSources`    | — | Persist the current dynamic source list to `SourcesFile` (JSON) |
+| `saveSources`    | — | Persist the dynamic source list **and** the calibration table to `SourcesFile`; replies `configSaved` |
+| `setCalibration` | `source` (label), `signal` (base name), `scale`, `offset`, `unit` | Record `value = raw × scale + offset` for one signal; metadata only, the hub never applies it. Identity entries are deleted. Replies with a `calibration` broadcast |
+| `reloadConfig`   | — | Re-read `SourcesFile`: calibration replaced wholesale, missing sources added, live sources never touched; replies `configReloaded` |
 | `getSources`     | — | Trigger `sources` broadcast |
 | `getConfig`      | `sourceId` | Trigger `config` broadcast for one source |
 | `getStats`       | — | Trigger `stats` broadcast |
@@ -402,8 +404,30 @@ binary frames carry data push payloads.
 | `triggerState`     | `state` (`"idle"`\|`"armed"`\|`"collecting"`\|`"triggered"`), `mode`, `stopped`, `trigTime?` | On any trigger FSM transition |
 | `zoom`             | `reqId`, `signals:{"src:sig":{t:[…], v:[…]}}` (`t` printed `%.17g`, `v` `%.9g`) | Unicast reply to `zoom` |
 | `maxPointsUpdated` | `maxPoints` | After ring buffer resize |
+| `calibration`      | `cal:[{source, signal, scale, offset, unit}]` | On connect; after an accepted `setCalibration`; after a successful `reloadConfig` |
+| `configSaved`      | `ok`, `path`, `error?` | In reply to `saveSources` |
+| `configReloaded`   | `ok`, `path`, `error?` | In reply to `reloadConfig` |
 | `pong`             | — | In reply to `ping` |
 
+### Config File Format
+
+`SourcesFile` is a flat JSON array of flat objects; `addr` marks a source,
+`signal` marks a calibration entry.
+
+```json
+[
+  {"label": "wave", "addr": "127.0.0.1:44500"},
+  {"source": "wave", "signal": "Adc", "scale": 0.00030518, "offset": -1.25, "unit": "V"}
+]
+```
+
+Flatness is a hard constraint: `StreamHub::LoadSourcesFile` scans from each `{`
+to the next `}`, so a nested object would truncate the parse. Both hubs read and
+write this format identically, and pre-calibration files load unchanged.
+
+Calibration is applied **client-side only**. Rings, history, `zoom` replies, both
+binary frames and the trigger comparator are all in raw units.
+
 ### Binary Push Frame (version 1, hub → client, binary WS frame)
 
 Little-endian throughout. Sent at `PushRate` Hz per source; contains **only

Docs/StreamHub-API.md

@@ -46,8 +46,57 @@ Reply (unicast): `{"type":"pong"}`.
 ```json
 {"type":"saveSources"}
 ```
-Persists the current dynamically-added source list to the hub's `SourcesFile`
+Writes the hub's `SourcesFile`: the current dynamically-added source list **and**
-(JSON array of `{label,addr,multicastGroup,dataPort}`); it is reloaded at startup.
+the calibration table, as one flat JSON array (see [§4](#4-config-file-format)).
+The hub replies with [`configSaved`](#configsaved). Despite the name, this
+command persists the whole config, not just the sources.
+
+### `setCalibration`
+
+```json
+{"type":"setCalibration","source":"wave","signal":"Adc","scale":0.00030518,"offset":-1.25,"unit":"V"}
+```
+Records an affine calibration `value = raw × scale + offset` for one signal,
+keyed by the source's **label** (not its runtime id) and the **base** signal
+name — one entry covers every element of an array signal.
+
+| Field | Type | Default | Validation |
+|---|---|---|---|
+| `source` | string | — | non-empty after trimming |
+| `signal` | string | — | non-empty after trimming; any trailing `[i]` is stripped |
+| `scale` | number | `1` | finite and non-zero |
+| `offset` | number | `0` | finite |
+| `unit` | string | `""` | trimmed, truncated to 16 chars; empty = use the streamer's own unit |
+
+Calibration is **metadata only**: the hub stores and redistributes it but never
+applies it. Ring buffers, recorded history, the `zoom` reply, both binary frames
+and the trigger comparator all stay in raw units — a client that ignores
+calibration behaves exactly as before.
+
+An entry that reduces to the identity (`scale = 1`, `offset = 0`, `unit = ""`) is
+**deleted** rather than stored, so a reset leaves no residue in the config file.
+
+On acceptance the hub broadcasts [`calibration`](#calibration) to every client. A
+rejected entry produces **no** broadcast, so the offending client reverts to the
+last value it was told.
+
+### `reloadConfig`
+
+```json
+{"type":"reloadConfig"}
+```
+Re-reads `SourcesFile` and then:
+
+- **replaces** the calibration table wholesale with the file's contents;
+- **adds** any source in the file that is not already active;
+- **never** removes, restarts or reconnects a live source.
+
+The asymmetry is deliberate: calibration is cheap to reapply, whereas a source is
+a live UDP session that must not be interrupted. An unsaved source the user added
+keeps streaming.
+
+The hub replies with [`configReloaded`](#configreloaded), followed on success by a
+`calibration` broadcast and a `sources` broadcast.
 
 ### `getSources` / `getConfig` / `getStats`
 
@@ -219,6 +268,37 @@ If history is not enabled: `{"type":"historyZoom","error":"history not enabled"}
 {"type":"maxPointsUpdated","maxPoints":50000}
 ```
 
+### `calibration`
+
+```json
+{"type":"calibration","cal":[
+  {"source":"wave","signal":"Adc","scale":0.00030518,"offset":-1.25,"unit":"V"}
+]}
+```
+The complete calibration table. Broadcast when a client connects (as an empty
+array when nothing is calibrated), after every accepted `setCalibration`, and
+after a successful `reloadConfig`. It is a separate frame rather than a field on
+`sources` because `sources` is serialised into a fixed 4 KiB buffer.
+
+### `configSaved`
+
+```json
+{"type":"configSaved","ok":true,"path":"/etc/streamhub/sources.json"}
+{"type":"configSaved","ok":false,"path":"","error":"no SourcesFile configured"}
+```
+Broadcast in reply to `saveSources`. `path` is always present (empty when the hub
+has no config file configured); `error` only when `ok` is false.
+
+### `configReloaded`
+
+```json
+{"type":"configReloaded","ok":true,"path":"/etc/streamhub/sources.json"}
+{"type":"configReloaded","ok":false,"path":"/etc/streamhub/sources.json","error":"cannot read sources file"}
+```
+Broadcast in reply to `reloadConfig`; same shape as `configSaved`. On success it
+is followed by a `calibration` broadcast and, if the file added any source, a
+`sources` broadcast.
+
 ---
 
 ## 3. Binary frames (hub → client)
@@ -270,7 +350,33 @@ per signal:
 
 ---
 
-## 4. Limits
+## 4. Config file format
+
+`SourcesFile` (C++ `SourcesFile` config key, Go `-sources-file` flag) is a flat
+JSON array of flat objects. A block containing `addr` is a source; a block
+containing `signal` is a calibration entry; anything else is skipped with a
+warning.
+
+```json
+[
+  {"label": "wave", "addr": "127.0.0.1:44500"},
+  {"label": "mc",   "addr": "127.0.0.1:44501", "multicastGroup": "239.0.0.1", "dataPort": 44502},
+  {"source": "wave", "signal": "Adc", "scale": 0.00030518, "offset": -1.25, "unit": "V"}
+]
+```
+
+**Every object must stay flat.** The C++ `StreamHub::LoadSourcesFile` parser
+takes each `{` up to the next `}` as one object, so a nested object anywhere in
+the file would truncate the parse at the inner brace. A nested
+`"calibration": {…}` inside a source entry is therefore not an option, and this
+is why calibration entries are siblings of sources rather than children.
+
+Files written by hub versions predating calibration load unchanged, and a file
+written by either hub loads in the other.
+
+---
+
+## 5. Limits
 
 | Limit | Value |
 |-------|-------|

Docs/WebUI.md

@@ -80,6 +80,21 @@ Signals received in the CONFIG packet are listed in the sidebar:
 - **Spatial arrays** — `TimeMode = PacketTime` arrays are shown as an expandable
   group; individual elements (`Ch1[0]`, `Ch1[1]`, …) can be dragged independently.
 
+The unit badge next to each signal shows the calibration's unit override when one
+is set, and the streamer's own unit otherwise.
+
+At the bottom of the sidebar, the collapsible **Sources & Config** section holds:
+
+- the `host:port`, label, multicast group and data port inputs plus **Connect**,
+  which adds a source at runtime;
+- **Save** — writes the source list and the whole calibration table to the hub's
+  config file;
+- **Reload** — re-reads that file. Calibration is replaced wholesale (so unsaved
+  edits are discarded), sources present in the file but not running are added,
+  and no running source is stopped or reconnected;
+- a status line showing the written path on success or the hub's error text on
+  failure.
+
 Click the sidebar toggle button (☰) to collapse/expand the signal list.
 
 ### Adding Plots
@@ -143,11 +158,31 @@ plot header showing per-signal vertical scale controls:
 | **V/div** | Volts (or units) per division |
 | **Pos (div)** | Screen position in divisions (draggable offset marker on Y axis) |
 | **Type** (Mixed mode only) | Toggle between **Analog** and **Digital** for this signal |
+| **Cal · Scale** | Data calibration gain. `value = raw × Scale + Offset` |
+| **Cal · Offset** | Data calibration bias, in calibrated units |
+| **Cal · Unit** | Overrides the unit reported by the streamer (max 16 chars) |
+| **Reset** | Clears this signal's calibration (`Scale = 1`, `Offset = 0`, no unit override) |
 | **✕** | Close the toolbar and deselect the signal |
 
 Offset markers (small triangles on the Y axis) show each signal's position and can
 be dragged to reposition signals without opening the toolbar.
 
+**Calibration vs. V/div and Offset.** They are different things. V/div and Offset
+are a *display* transform: they move and stretch the trace on screen. Calibration
+changes *the value itself* — the plot, the Y-axis tick labels, the cursor and
+hover readouts, the CSV export and the trigger threshold all report
+`raw × Scale + Offset` in the calibrated unit. V/div is then read as "calibrated
+units per division" and Offset as "the calibrated value at screen centre".
+
+The calibration header names the **base** signal and its element count, because
+one entry covers every element of an array — opening the toolbar on `Adc[3]` and
+editing the calibration moves all of `Adc`.
+
+Calibration is keyed by the source's **label**, is shared with every other
+browser connected to the same hub, and is not persisted until you press **Save**
+in the Sources & Config section. It is mirrored to `localStorage` so it survives
+a page reload even against a hub with no config file.
+
 ### Plot Controls
 
 | Control | Action |

docs/superpowers/specs/2026-08-16-udpstreamer-signal-calibration-and-config-design.md

@@ -193,7 +193,8 @@ change never sends `calibration`, so the mirror simply remains authoritative;
 the same holds for a hub started without a config file.
 
 **Validation.** The same rules as the hub are enforced in the input handlers: a
-non-finite or zero `scale` reverts the field to its last accepted value.
+non-finite or zero `scale` keeps the rejected text in the field and marks it
+with a red `cal-invalid` border, so the user can see what was wrong.
 
 ## Testing