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>

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 |