M8: local screens (IP + Loading) + BOOT button

local_screens.{h,c}:
- Embedded 8x8 bitmap font sized to a 95-char table; only the glyphs
  used by today's strings (viewport.local, the IPv4 string, Loading...)
  are populated. Unsupported chars render blank.
- 768 KB PSRAM scratch FB at panel-native dims.
- local_screens_show_ip() — two centered lines:
    "viewport.local"
    <current IP>
  Scale 3x portrait / 4x landscape; centered vertically.
- local_screens_show_loading() — centered "Loading..." at scale 4x/5x.
- local_screens_restore_for_state() — repaint after BOOT-overlay expiry:
  UNCONFIGURED -> IP screen; AWAKE/ASLEEP -> black FB (next /frame or
  sleep handles the rest).
- All paths go through display_present_rgb565() so orientation rotation
  is automatic.

button.{h,c}:
- Polling task at 30ms, active-low with internal pull-up.
- Short press (<5s, released) → backlight on, IP overlay for 15s via
  esp_timer one-shot, then restore.
- Hold ≥ 5s → nvs_config_reset() + esp_restart().
- PIN_BOOT_BUTTON = GPIO 0 is a TODO placeholder. ESP32-P4 strap pin
  GPIO35 is owned by RMII TXD1 at runtime so Waveshare must expose a
  separate user button on a free pin; confirm against the schematic.
  Fail-soft if mis-wired: input reads stuck-high and the task never
  fires.

state_machine on AWAKE transition now calls local_screens_show_loading()
right after display_wake() — so every wake (tap or POST /state) flashes
a clear "Loading..." until the next /frame paints over it.

app_main on boot:
- Calls local_screens_init() after display_init().
- UNCONFIGURED → paint IP screen (replaces the M3 test pattern).
- ASLEEP → backlight off (unchanged).
- After all subsystems: button_init().

Build clean against ESP-IDF 5.4 (binary ~870 KB; the font/glyph data is
under 800 bytes).

TESTING.md M8 documents the visual checks, the BOOT-pin placeholder
caveat, the AWAKE-overlay-overwrite behavior, the open follow-up about
post-overlay backlight when ASLEEP, and the font fallback policy.

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