test(dev): emulation + linting harness (Wokwi, native tests, CI, docs)

Answers "how do we emulate/lint this?" with runnable tooling for every part, and
factors the pure firmware logic out so it is unit-testable on the host.

What changed:
- firmware/src/wav.h, firmware/src/httprange.h: pure, header-only WAV header builder
  and HTTP Range parser (no Arduino deps), so the tricky byte-layout and range logic
  can be tested on a PC. storage.cpp and api_http.cpp refactored to use them.
- firmware/test/test_pure/: Unity tests for the WAV header fields and Range parsing
  (start-end, open-ended, clamped, unsatisfiable, non-bytes). Run: pio test -e native.
- firmware/platformio.ini: add [env:native] (host tests) and [env:esp32s3_wokwi]
  (emulator build with default WiFi = Wokwi-GUEST so the API comes up in the sim).
- firmware/wokwi.toml + firmware/diagram.json: Wokwi emulator harness (ESP32-S3 +
  microSD + button + LED). Note: Wokwi has no I2S mic part, so audio isn't emulated;
  the harness targets boot + WiFi + REST API.
- firmware/src/main.cpp + config.cpp: bring up WiFi + API even if audio/SD init fails
  (device stays reachable, reports the fault via GET /device); compile-time default
  WiFi honoured when NVS is empty (used by the Wokwi build).
- .forgejo/workflows/ci.yml: add native tests + cppcheck to the firmware job; new
  openapi job (openapi-spec-validator) and emulator job (Wokwi build, plus a full run
  when WOKWI_CLI_TOKEN is set).
- docs/testing.md: the full emulate/lint guide for firmware, server, API, app, case.
- state/: NOTES points at the guide; TODO reflects this branch.

Why:
- The firmware can't be flashed yet (no parts) and doesn't build on this dev host, so
  we need host-runnable checks. Pure-logic unit tests + OpenAPI validation run anywhere;
  Wokwi emulates boot/WiFi/API; CI compiles the real firmware. Verified locally: the
  OpenAPI spec validates (12 paths, 10 schemas).

Notes:
- Native tests and cppcheck run in CI (no compiler on the dev host). The Wokwi full run
  is skipped unless a WOKWI_CLI_TOKEN secret is present; the build is still verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/diagram.json

@@ -0,0 +1,28 @@
+{
+  "version": 1,
+  "author": "OpenScribe",
+  "editor": "wokwi",
+  "comment": "ESP32-S3 + microSD + button + LED. Wokwi has no I2S MEMS mic part, so audio capture is not emulated; this harness exercises boot, WiFi and the REST API (and SD where supported). See ../docs/testing.md.",
+  "parts": [
+    { "type": "board-esp32-s3-devkitc-1", "id": "esp", "top": 0, "left": 0, "attrs": {} },
+    { "type": "wokwi-microsd-card", "id": "sd1", "top": -120, "left": 150, "attrs": {} },
+    { "type": "wokwi-pushbutton", "id": "btn1", "top": 120, "left": 150, "attrs": { "color": "green" } },
+    { "type": "wokwi-led", "id": "led1", "top": 120, "left": -100, "attrs": { "color": "blue" } },
+    { "type": "wokwi-resistor", "id": "r1", "top": 120, "left": -180, "attrs": { "value": "330" } }
+  ],
+  "connections": [
+    [ "esp:GND.1", "sd1:GND", "black", [] ],
+    [ "esp:3V3.1", "sd1:VCC", "red", [] ],
+    [ "esp:12", "sd1:CLK", "yellow", [] ],
+    [ "esp:11", "sd1:DI", "green", [] ],
+    [ "esp:13", "sd1:DO", "blue", [] ],
+    [ "esp:10", "sd1:CS", "orange", [] ],
+
+    [ "esp:7", "btn1:1.l", "purple", [] ],
+    [ "btn1:2.l", "esp:GND.2", "black", [] ],
+
+    [ "esp:48", "r1:1", "blue", [] ],
+    [ "r1:2", "led1:A", "blue", [] ],
+    [ "led1:C", "esp:GND.3", "black", [] ]
+  ]
+}