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/08-payload-group.md

@@ -0,0 +1,81 @@
+# MeshCore Protocol Specification
+
+## Section 8: Payload — Group (GRP_TXT / GRP_DATA)
+
+### Overview
+
+Group payloads enable encrypted communication within a channel. All members of a
+channel share a symmetric secret. The payload begins with a 1-byte channel hash
+(derived from the channel's shared key), followed by an encrypt-then-MAC blob.
+
+### Payload Types
+
+| Value | Name | Description |
+|-------|------|-------------|
+| 0x05 | GRP_TXT | Group text message |
+| 0x06 | GRP_DATA | Group datagram (binary data) |
+
+### Wire Format
+
+```
+ 0
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|channel_hash(1)|    Cipher MAC (2 bytes)       |               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
+|              Ciphertext (N × 16 bytes, N ≥ 1)                |
+|                                                               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
+
+### Fields
+
+| Field | Offset | Size | Type | Description |
+|-------|--------|------|------|-------------|
+| channel_hash | 0 | 1 byte | raw | First byte of SHA-256(channel_shared_key) |
+| cipher_mac | 1 | 2 bytes | raw | Truncated HMAC-SHA256 over ciphertext |
+| ciphertext | 3 | variable (multiple of 16) | raw | AES-128-ECB encrypted data |
+
+### Minimum Size
+
+- Minimum payload: 1 + 2 + 16 = 19 bytes
+
+### Channel Hash Derivation
+
+The channel_hash is the first byte of the SHA-256 hash of the channel's 32-byte
+shared key (PUB_KEY_SIZE bytes):
+
+```
+channel_hash = SHA256(channel_shared_key)[0]
+```
+
+This means hash collisions are expected (1 in 256 chance). When a receiver finds
+a matching channel_hash, it attempts decryption with each matching channel's
+secret. Only a successful MAC verification confirms the correct channel.
+
+### Encryption
+
+1. Encrypt plaintext using AES-128-ECB with first 16 bytes of the channel's
+   shared key (GroupChannel.secret).
+2. Compute HMAC-SHA256 over ciphertext using all 32 bytes of the channel's
+   shared key. Truncate to 2 bytes.
+3. Assemble: channel_hash || MAC || ciphertext.
+
+### Decryption
+
+1. Read channel_hash (1 byte).
+2. Search local channel database for channels with matching hash. Multiple
+   channels may match (up to 4 in reference implementation).
+3. For each matching channel, attempt MAC verification and decryption using the
+   channel's shared key.
+4. The first channel that produces a valid MAC yields the correct decryption.
+
+### Cross-References
+
+- [Section 14: Cryptography](14-crypto.md) — Encrypt-then-MAC
+- Test vectors: `corpus/payloads/group/`
+
+### Reference Implementation
+
+- `Mesh::createGroupDatagram()` in `src/Mesh.cpp` — Encoding
+- `Mesh::onRecvPacket()`, cases `PAYLOAD_TYPE_GRP_TXT/GRP_DATA` — Decoding
+- `Mesh::searchChannelsByHash()` — Channel lookup