fix: address all 9 findings from final calibration code review

- calibration.js: fix baseSignalName('[0]') parity with Go/C++ (>= 0 not > 0)
- calibration.test.js: add assertions for '[0]' edge case in two existing tests
- app.js: remove stale typeof guard around refreshVScaleMenu (always defined)
- app.js: call refreshTrigThresholdField on trig-signal change (both assignment sites)
- index.html: drop maxlength='16' on unit input; normaliseCal is the sole enforcer
- configcheck/main.go: delete dead nextOneOf function (no callers)
- hub_calibration_test.go: delete orphaned waitBroadcast comment (function never existed)
- calibration.go: correct arrayIndexSuffix comment to document known Go/C++ difference
- Docs/StreamHub-API.md: add calibration entry count and unit byte limits to §5 table
- spec: fix configReloaded missing path field, '16 chars'→'16 UTF-8 bytes', StreamString→char[], chain scenario→configcheck program, four→five new frames

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

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

@@ -44,7 +44,7 @@ A calibration entry is keyed by `(source label, signal base name)`:
 | `signal` | string | — | base signal name, no `[i]` suffix |
 | `scale` | float64 | `1` | finite, non-zero |
 | `offset` | float64 | `0` | finite |
-| `unit` | string | `""` | trimmed, max 16 chars; empty means "use the streamer's unit" |
+| `unit` | string | `""` | trimmed, max 16 UTF-8 bytes; empty means "use the streamer's unit" |
 
 The key uses the source **label**, not the runtime id (`s1`, `s2`). Ids are
 assigned in add-order at startup, so a saved calibration keyed by id would rebind
@@ -90,7 +90,7 @@ New frames, implemented identically in both hubs:
 | client → hub | `{"type":"setCalibration","source","signal","scale","offset","unit"}` |
 | hub → client | `{"type":"configSaved","ok":bool,"path":string,"error":string}` |
 | client → hub | `{"type":"reloadConfig"}` |
-| hub → client | `{"type":"configReloaded","ok":bool,"error":string}` |
+| hub → client | `{"type":"configReloaded","ok":bool,"path":string,"error":string}` |
 
 `calibration` is broadcast when a client connects and after every accepted
 `setCalibration` or successful `reloadConfig`. It is a separate message rather
@@ -128,9 +128,10 @@ sibling `CalConfig` type; `Save` writes both slices into one array;
 `Load` decodes into `[]map[string]json.RawMessage` and discriminates per element.
 
 **C++ (`Source/Applications/StreamHub/StreamHub.{h,cpp}`).** A fixed
-`kMaxCalibration = 256` array of
-`{StreamString source, signal, unit; float64 scale, offset;}` — no STL, per the
-`Source/Components` and StreamHub style rules. `HandleSetCalibration`,
+`kMaxCalibration = 256` array of `CalibrationEntry` structs with fixed
+`char[]` fields (`source[128]`, `signal[128]`, `unit[17]`) and `float64` scale
+and offset — no STL and no per-entry heap allocation, per the `Source/Components`
+and StreamHub style rules. `HandleSetCalibration`,
 `HandleReloadConfig` and `BroadcastCalibration` mirror the existing
 `HandleAddSource` / `BroadcastSources` shape, with `BroadcastCalibration` using
 its own 16 KiB buffer like `BroadcastConfig`. `LoadSourcesFile` gains the
@@ -203,10 +204,12 @@ new-format file, unrecognised block, malformed entry), `setCalibration`
 validation including `scale = 0` and non-finite values, and a save→load
 round-trip asserting sources and calibration both survive.
 
-**C++.** Extend an E2E `chain` scenario's config file with a calibration entry and
-assert the hub re-serialises it unchanged after a `saveSources`, which exercises
-the discriminator branch in `LoadSourcesFile` and the writer in
-`HandleSaveSources`.
+**C++.** A standalone Go program at `Test/E2E/suite/client/configcheck/` connects
+to either hub (Go or C++) via WebSocket and asserts identical calibration
+behaviour: it exercises `setCalibration`, `saveSources → configSaved`,
+`reloadConfig → configReloaded`, and verifies that the saved config file and all
+broadcast frames are sorted and complete.  The program exits non-zero on any
+deviation from the protocol, making it runnable against both hubs in CI.
 
 **Browser.** `node --check static/app.js`, plus a manual pass: set a scale and
 offset on a live signal and confirm the plot, hover readout, cursor readout, CSV
@@ -216,6 +219,7 @@ the live source keeps streaming.
 
 ## Documentation
 
-`Docs/StreamHub-API.md` gains the four new frames; `Docs/WebUI.md` gains the
+`Docs/StreamHub-API.md` gains the five new frames (`calibration`, `setCalibration`,
+`configSaved`, `reloadConfig`, `configReloaded`); `Docs/WebUI.md` gains the
 calibration row and the Sources & Config section; `ARCHITECTURE.md` §6 gains the
 config file format.