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>

corpus/crypto/sha256/basic.json

@@ -1,7 +1,7 @@
 {
   "$schema": "../../schema/test-vector.schema.json",
   "name": "SHA-256 Basic",
-  "spec_ref": "spec/14-crypto.md",
+  "spec_ref": "src/14-crypto.md",
   "protocol_version": 0,
   "description": "Basic SHA-256 hash computation test vectors. MeshCore uses SHA-256 for packet hashing, channel hash derivation, and HMAC.",
   "vectors": [