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>

state/TODO.md

@@ -5,8 +5,7 @@
 
 ## In progress
 
-- [ ] M2 On-device WiFi + REST API - code complete on `feature/fw-rest-api`, in PR.
-      Not compiled locally / hardware-verified; CI builds both profiles.
+- [ ] Dev testing + emulation harness - on `feature/dev-testing`, in PR.
 
 ## Pending (roughly in build order)