M6: state machine — POST /state, idle timer, /frame 409 guard

state_machine.{h,c} — central wake/sleep transitions:
- state_machine_init() creates the esp_timer one-shot for the idle timer.
- state_machine_set(target) is idempotent and atomic. On AWAKE: backlight
  on, idle timer (re)armed. On ASLEEP: idle timer cancelled, backlight off.
  Rejects with INVALID_STATE when the device is unconfigured.
- state_machine_frame_painted() restarts the idle timer if awake; called
  by /frame after each successful paint.
- Idle-timer callback transitions to ASLEEP. TODO M7 hook: outbound POST
  {viewport, state:sleep} to <scrypted>/state.

http_api.c:
- POST /state: parse {state}, accept "wake"/"sleep", reject others 400.
  Unconfigured device → 409 "device unconfigured". Already-in-state → 204
  (idempotent no-op). Successful transition → 204.
- POST /frame: 409 Conflict when state != AWAKE. After successful paint,
  call state_machine_frame_painted() so the idle clock keeps resetting
  while frames stream.

app_main:
- Initialize state_machine before http_api so the route handler can drive
  it from request 0.
- After display_init(), reconcile the panel with the boot state:
    UNCONFIGURED → test pattern (placeholder until M8 IP screen)
    ASLEEP       → display_sleep() so a configured device boots dark
    AWAKE        → leave on (not reached on fresh boot)

Disable path: idle_timeout_ms=0 in /config means the timer is never armed
and a wake state persists until /state {sleep} or a power cycle.

Build clean against ESP-IDF 5.4 (binary ~645 KB).

TESTING.md M6 expands with idempotency checks, 409-when-asleep, 409-when-
unconfigured, idle-timer firing within idle_timeout_ms+slack, /frame
restarting the idle timer, idle-timer disable via idle_timeout_ms=0, and
the M7 dependency note about the missing outbound sleep POST.

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