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>

README.md

@@ -3,6 +3,8 @@
 A comprehensive specification and conformance test suite for the
 [MeshCore](https://github.com/meshcore-dev/MeshCore) mesh networking protocol.
 
+**📖 Read the spec online:** <https://swaits.github.io/meshcore-spec/>
+
 MeshCore Spec aims to be the authoritative, machine-readable definition of the
 MeshCore wire format — much like [RubySpec](https://github.com/ruby/spec) became
 the definitive specification of the Ruby language. Any implementation that passes
@@ -38,12 +40,17 @@ before relying on the spec for anything load-bearing.
 
 ## What's Here
 
-- **`spec/`** — RFC-style protocol specification in Markdown, covering every
-  layer from packet framing to cryptographic operations.
+- **`src/`** — RFC-style protocol specification in Markdown, covering every
+  layer from packet framing to cryptographic operations. Also the source root
+  for the [mdBook](https://rust-lang.github.io/mdBook/) build published to
+  GitHub Pages.
 - **`corpus/`** — JSON test vectors organized by protocol layer.
   Each vector pairs a structured representation with its exact binary encoding,
   enabling any implementation to verify encode and decode correctness.
 - **`tools/`** — Validation and generation utilities.
+- **`book.toml`** + **`.github/workflows/mdbook.yml`** — mdBook configuration
+  and the GitHub Actions workflow that builds and deploys the spec on every
+  push to `main`.
 
 ## Test Vector Format