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/12-payload-control.md

@@ -0,0 +1,68 @@
+# MeshCore Protocol Specification
+
+## Section 12: Payload — Control
+
+### Overview
+
+The CONTROL payload carries control and discovery data. Control packets are raw
+byte payloads whose structure is determined by the first byte. A subset of
+control packets (those with bit 7 of the first byte set) are restricted to
+zero-hop delivery only.
+
+### Payload Type
+
+Header payload type field: `0x0B` (CONTROL)
+
+### Wire Format
+
+```
+ 0
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|  Control Byte |  Data...      |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
+
+### Fields
+
+| Field | Offset | Size | Type | Description |
+|-------|--------|------|------|-------------|
+| control_byte | 0 | 1 byte | uint8 | Control type identifier |
+| data | 1 | variable | raw | Control-specific data |
+
+### Zero-Hop Restriction
+
+When bit 7 (`0x80`) of the control byte is set, the packet is restricted to
+zero-hop delivery only. The reference implementation enforces this:
+
+- If `(payload[0] & 0x80) != 0` AND the packet is direct-routed:
+  - It is processed only if `getPathHashCount() == 0`.
+  - It is NOT forwarded to other nodes.
+
+### Discovery Protocols
+
+Control packets are commonly used for node discovery. The specific sub-protocol
+formats are application-defined, but typically include:
+
+- **Discovery Request**: Sent zero-hop to find nearby nodes
+- **Discovery Response**: Contains node type, name, SNR, and other metadata
+
+### Encoding
+
+1. Construct the raw control data bytes.
+2. If the control packet should be zero-hop only, set bit 7 of the first byte.
+3. Copy data to payload, set payload_len.
+
+### Constraints
+
+- Payload MUST be at least 1 byte (the control byte).
+- Maximum payload: 184 bytes (MAX_PACKET_PAYLOAD).
+
+### Cross-References
+
+- [Section 17: Routing](17-routing.md) — Zero-hop delivery
+- Test vectors: `corpus/payloads/control/`
+
+### Reference Implementation
+
+- `Mesh::createControlData()` in `src/Mesh.cpp` — Encoding
+- `Mesh::onRecvPacket()`, CONTROL handling — Zero-hop check and delivery