feat(mdbook): publish spec via mdBook on GitHub Pages

Set up the protocol spec as an mdBook and add a GitHub Actions
workflow that builds it on every push/PR and deploys to GitHub
Pages on main. The repo is now both a machine-readable corpus and
a browsable, searchable site.

Structure changes (one-time, mechanical):

- Rename the spec source directory from `spec/` to `src/` to match
  mdBook's default convention. All 21 chapter files move; their
  internal relative cross-links are unchanged.
- Add `src/SUMMARY.md` as the mdBook table of contents, grouping
  chapters under Wire Format, Payloads, Cryptography & Identity,
  and Routing & Higher-Layer Protocols.
- Add `book.toml` at repo root: title, author, search enabled,
  fold enabled, edit-on-GitHub links, navy theme.
- Add `.github/workflows/mdbook.yml`: build on push + PR, deploy
  on push to main only (configure-pages + upload-pages-artifact +
  deploy-pages from actions/).
- Add `/book/` to .gitignore.

Reference updates so the old path is fully retired:

- Every `spec_ref` in `corpus/**/*.json` rewritten from `spec/` to
  `src/` (54 files).
- Schema description text and example path in
  `corpus/schema/test-vector.schema.json`.
- `README.md`: prominent link to the published site near the top;
  "What's Here" updated for the new layout.
- `CONTRIBUTING.md`: spec-editing instructions point at `src/`
  and note the SUMMARY.md requirement.
- `CLAUDE.md`: directory-layout notes updated.

CHANGELOG.md historical entries intentionally keep their `spec/`
paths since those record what the paths were at the time of those
commits.

After this lands, the user needs to enable GitHub Pages in repo
settings with source = "GitHub Actions" (one-time, manual). The
workflow will then publish to https://swaits.github.io/meshcore-spec/
on every push to main.

Verified locally: `mdbook build` produces a clean book/ tree with
all 21 chapters rendered and the SUMMARY-driven sidebar populated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/18-companion-protocol.md

@@ -0,0 +1,96 @@
+# MeshCore Protocol Specification
+
+## Section 18: Companion Protocol
+
+### Overview
+
+The companion protocol enables BLE (Bluetooth Low Energy) and serial
+communication between a MeshCore radio and a companion app (phone, tablet, or
+computer). It uses a binary framing format over the Nordic UART Service (NUS)
+BLE profile.
+
+### BLE Service
+
+| UUID | Description |
+|------|-------------|
+| `6E400001-B5A3-F393-E0A9-E50E24DCCA9E` | Nordic UART Service |
+| `6E400002-...` | RX Characteristic (app writes to radio) |
+| `6E400003-...` | TX Characteristic (radio notifies app) |
+
+### Frame Format
+
+Each companion protocol frame consists of a 1-byte type identifier followed by
+variable-length data:
+
+```
+[type(1)][data(0-171)]
+```
+
+Maximum frame size is 172 bytes.
+
+### Message Types (Radio to App)
+
+| Type | Value | Description |
+|------|-------|-------------|
+| PACKET_CHANNEL_MSG_RECV | 0x01 | Channel message received |
+| PACKET_CONTACT_MSG_RECV | 0x02 | Contact message received |
+| PACKET_ADV_RECV | 0x03 | Advertisement received |
+| PACKET_CONTACT_MSG_RECV_V3 | 0x04 | Contact message V3 (includes SNR) |
+| PACKET_CHANNEL_MSG_RECV_V3 | 0x05 | Channel message V3 (includes SNR) |
+
+### Command Types (App to Radio)
+
+| Command | Value | Description |
+|---------|-------|-------------|
+| CMD_APP_START | 0x01 | Initialize connection |
+| CMD_DEVICE_QUERY | 0x02 | Query device info |
+| CMD_SET_CHANNEL | 0x03 | Set active channel |
+| CMD_SEND_CHANNEL_MESSAGE | 0x04 | Send message to channel |
+| CMD_SEND_CONTACT_MESSAGE | 0x05 | Send message to contact |
+| CMD_GET_CONTACTS | 0x06 | Get contact list |
+| CMD_GET_CHANNELS | 0x07 | Get channel list |
+| CMD_SET_TIME | 0x08 | Set device time |
+| CMD_ADD_CONTACT | 0x09 | Add a contact |
+| CMD_ADD_CHANNEL | 0x0A | Add a channel |
+| CMD_SEND_LOGIN | 0x0B | Login to room/repeater |
+| CMD_GET_SETTINGS | 0x0C | Get device settings |
+| CMD_SET_SETTINGS | 0x0D | Set device settings |
+| CMD_REMOVE_CONTACT | 0x0E | Remove a contact |
+| CMD_REMOVE_CHANNEL | 0x0F | Remove a channel |
+| CMD_SHARE_CONTACT | 0x10 | Share contact info |
+| CMD_SET_ADV_NAME | 0x11 | Set advertisement name |
+| CMD_REBOOT | 0x12 | Reboot device |
+| CMD_SEND_RAW | 0x13 | Send raw packet |
+
+### Connection Sequence
+
+1. Scan for BLE devices advertising the Nordic UART Service.
+2. Connect and discover services.
+3. Enable notifications on the TX characteristic.
+4. Send `CMD_APP_START` to initialize.
+5. Send `CMD_DEVICE_QUERY` to get device info.
+6. Send `CMD_SET_TIME` to synchronize clock.
+7. Send `CMD_GET_CONTACTS` and `CMD_GET_CHANNELS` to fetch state.
+
+### Byte Order
+
+All multi-byte integers in the companion protocol are **little-endian**, except
+for CayenneLPP sensor data which uses **big-endian**.
+
+### Message Length Limit
+
+Text messages are limited to 133 characters.
+
+### MTU Considerations
+
+The default BLE MTU is 23 bytes (20 bytes of payload). For complex operations,
+implementations SHOULD request a larger MTU (up to 512 bytes).
+
+### Cross-References
+
+- [Section 1: Wire Format](01-wire-format.md) — Underlying packet format
+- Test vectors: `corpus/companion/`
+
+### Reference Implementation
+
+- `docs/companion_protocol.md` in the MeshCore repository