feat: replace JSON BLE protocol with compact binary encoding

[spectrachrome]

Summary

• Replace the JSON-over-NUS BLE protocol with a compact binary protocol
  (spec: docs/binary_protocol.md) — state snapshots shrink from ~300 bytes
  / ~15 BLE packets down to 22 bytes (1–2 packets)
• Setter commands are 1–4 bytes with single-byte ack responses (0x00 OK /
  0xE0 parse error / 0xE1 range error), replacing verbose JSON
  round-trips
• Add TestPattern command (0xF0) that triggers a 5-second solid-color
  episode (red/green/blue/white) at full intensity, bypassing all
  post-processing — useful for verifying LED wiring and color order
• Add GetVersion command (0x02) returning protocol version + firmware
  semver (5 bytes)
• Remove serde, serde_json_core, and heapless crate dependencies
  (saves ~15 KB flash)

What changed

File	Change
src/ble.rs	Full rewrite: binary command parser
(handle_binary_command), state encoder (22-byte snapshot), version encoder, constants for all command/response IDs, unit tests
src/bin/main.rs	Shrink TX buffer 512→32 bytes, remove RX reassembly buffer entirely, remove newline framing, binary ble_handle_message, wall-clock test pattern deadline, BLE RX logging
src/state.rs	Add TEST_PATTERN signal for inter-task test pattern
trigger
Cargo.toml	Remove serde, serde_json_core, heapless
docs/binary_protocol.md	Full protocol specification (frame format,
command table, state snapshot layout, version response, error codes)

Protocol at a glance

App → Device (commands):
0x01 GetState → 22-byte state snapshot
0x02 GetVersion → 5-byte version response
0x10..0x1D Setters → 1-byte ack (0x00 OK)
0xF0 TestPattern → 1-byte ack, 5s solid color episode

Device → App (responses):
0x00 OK
0x01 + 21B State snapshot
0x02 + 4B Version (protocol v1, fw semver)
0xE0 Parse error
0xE1 Range error

Test plan

• Flash firmware, connect with nRF Connect — verify GetState (0x01)
  returns 22 bytes starting with 0x01 0x01
• Send GetVersion (0x02) — verify 5-byte response 02 01 00 01 00
• Send SetBrightness (10 80) — verify 00 ack, then GetState
  shows updated brightness
• Send TestPattern red (F0 00) — verify 5-second solid red, returns
  to normal pattern
• Send out-of-range value (e.g. 12 00 for FPS=0) — verify E1 range
  error
• Verify state auto-push on flight mode change (arm/disarm with FC
  connected)
• Verify strobe only activates when armed/arming-allowed with TX linked