feat(mdbook): show spec version in overview body + page title

The first version cut (v0.1.0) revealed a real gap: nothing in the
rendered book identified which release the reader was on. The
version-picker chip says "version: v0.1.0" when you're on /v0.1.0/,
but it's a small UI affordance — and on /latest/ it can't say
"v0.1.0" because /latest/ is the rolling main build, not the v0.1.0
release.

Two complementary fixes, both visible without JS:

1. **Body marker.** Top of every chapter-0 (overview) page now reads
   `Spec version: <token>`. The token is `latest (main)` in src/, and
   gets frozen to the actual version (`v0.1.0`) when cut-version
   snapshots the directory. So /latest/ overview says "latest (main)"
   and /v0.1.0/ overview says "v0.1.0".

2. **Per-build page title.** scripts/build-site.sh now sets
   `MDBOOK_BOOK__TITLE` for each build:
     - latest → "MeshCore Protocol Specification — latest (main)"
     - vX.Y.Z → "MeshCore Protocol Specification — vX.Y.Z"
   The browser tab and mdBook sidebar header reflect the version.

`just cut-version` updated to run a one-line sed on the snapshot's
00-overview.md replacing the `latest (main)` marker. Defensive grep
warns if the marker isn't found (e.g., src/ was edited away from
the expected line).

versions/v0.1.0/00-overview.md regenerated by re-running
`just cut-version v0.1.0` after the marker landed in src/ — so the
v0.1.0 snapshot now self-identifies correctly.

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

justfile

@@ -51,6 +51,14 @@ cut-version VERSION:
     fi
     mkdir -p versions
     cp -R src "versions/{{VERSION}}"
+    # Freeze the version marker in the snapshot. src/ keeps the literal
+    # `latest (main)` so the rolling /latest/ build stays accurate.
+    sed -i 's|`latest (main)`|`{{VERSION}}`|' "versions/{{VERSION}}/00-overview.md"
+    if ! grep -q '`{{VERSION}}`' "versions/{{VERSION}}/00-overview.md"; then
+        echo "warning: expected version marker not found in 00-overview.md; the" >&2
+        echo "snapshot still says 'latest (main)'. Check that src/00-overview.md" >&2
+        echo "contains the marker line ('**Spec version:** \`latest (main)\`')." >&2
+    fi
     echo "Snapshotted src/ -> versions/{{VERSION}}"
     echo
     echo "Next steps:"