feat(mdbook): switch versioning from git tags to versions/ directories

Replace the git-tag-based snapshot model with frozen-directory
snapshots. Each released spec version is its own subtree under
./versions/vX.Y.Z/ — a literal copy of ./src/ taken at release time.

Why: tagging the whole repo conflated *spec content* with *site
infrastructure* (book.toml, theme, picker JS, workflow, build
scripts). Improving any of those required cherry-picking the change
back to every old tag. With directory snapshots, content is frozen
but every deploy renders every version with TODAY's infrastructure
— site fixes propagate everywhere for free.

Changes:

- scripts/build-site.sh now iterates ./versions/v* directories
  instead of `git tag --list 'v*'`. Each per-version build sets
  MDBOOK_BOOK__SRC to the snapshot dir and MDBOOK_OUTPUT__HTML__SITE_URL
  to /meshcore-spec/<version>/. The root book.toml, theme, and
  picker JS apply uniformly.
- justfile gains `cut-version vX.Y.Z` — validates the name, refuses
  to overwrite, runs `cp -R src versions/<version>`, prints next-
  step hints. Cutting a release is now `just cut-version v0.1.0`,
  then a commit; optionally tag the commit for provenance, but
  tags no longer drive CI.
- .github/workflows/mdbook.yml drops the `tags: ['v*']` trigger
  and `fetch-depth: 0` (no longer iterating git history). Triggers
  on push to main and PRs.
- src/00-overview.md: "Spec Versions" section rewritten to describe
  the directory-snapshot model and the deliberate content-vs-
  infrastructure split.

Smoke-tested locally: `just build-site` with no versions/ produces
`_site/{latest/, versions.json=["latest"], index.html}`.
`just cut-version v0.0.1` + rebuild produces both `latest/` and
`v0.0.1/`, versions.json=`["latest","v0.0.1"]`.

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

justfile

@@ -35,3 +35,24 @@ clean:
 # Run the corpus validator.
 validate:
     python3 tools/validate.py corpus/
+
+# Cut a frozen spec version: copy ./src/ to ./versions/<VERSION>/.
+# Usage: just cut-version v0.2.0
+cut-version VERSION:
+    #!/usr/bin/env sh
+    set -eu
+    case "{{VERSION}}" in
+        v[0-9]*.[0-9]*.[0-9]*) ;;
+        *) echo "Version must look like vX.Y.Z (got '{{VERSION}}')" >&2; exit 1 ;;
+    esac
+    if [ -e "versions/{{VERSION}}" ]; then
+        echo "versions/{{VERSION}} already exists; not overwriting." >&2
+        exit 1
+    fi
+    mkdir -p versions
+    cp -R src "versions/{{VERSION}}"
+    echo "Snapshotted src/ -> versions/{{VERSION}}"
+    echo
+    echo "Next steps:"
+    echo "  - git add versions/{{VERSION}} && commit (e.g. \"release {{VERSION}}\")"
+    echo "  - optionally tag the commit: git tag {{VERSION}}"