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>

versions/v0.1.0/00-overview.md

@@ -2,6 +2,9 @@
 
 ## Section 0: Overview
 
+**Spec version:** `v0.1.0` · see
+[Spec Versions](#spec-versions) for the full list of frozen releases.
+
 ### Status and Provenance
 
 > **This is an unofficial, independent specification.** It is **not** part of