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/src/httprange.h

@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-3.0-only
+// Pure, hardware-independent HTTP Range header parser. Header-only so it compiles both on
+// the ESP32 and on the host for native unit tests (see firmware/test, docs/testing.md).
+#pragma once
+
+#include <stddef.h>
+
+namespace oshttp {
+
+struct RangeResult {
+  bool partial;       // a "bytes=" range was requested
+  bool satisfiable;   // start is within the file
+  size_t start;       // inclusive
+  size_t end;         // inclusive
+};
+
+// Parse a Range header value like "bytes=START-END" (END optional) against a known total
+// size. If header is null/empty or not a bytes range, returns partial=false (send full
+// body). A start past the end sets satisfiable=false (caller sends 416).
+inline RangeResult parseByteRange(const char *header, size_t total) {
+  RangeResult r{false, true, 0, total ? total - 1 : 0};
+  if (!header) return r;
+
+  const char *prefix = "bytes=";
+  for (size_t i = 0; prefix[i]; ++i) {
+    if (header[i] != prefix[i]) return r;  // not a bytes range -> full body
+  }
+  r.partial = true;
+
+  const char *s = header + 6;
+  size_t start = 0;
+  while (*s >= '0' && *s <= '9') { start = start * 10 + (*s - '0'); ++s; }
+  if (*s != '-') { r.partial = false; return r; }  // malformed -> full body
+  ++s;
+
+  size_t end = total ? total - 1 : 0;
+  bool anyEnd = false;
+  size_t ev = 0;
+  while (*s >= '0' && *s <= '9') { ev = ev * 10 + (*s - '0'); ++s; anyEnd = true; }
+  if (anyEnd) end = ev;
+
+  if (start >= total) { r.satisfiable = false; r.start = start; r.end = end; return r; }
+  if (end >= total) end = total ? total - 1 : 0;
+  r.start = start;
+  r.end = end;
+  return r;
+}
+
+}  // namespace oshttp