feat(engine,deploy): seed-drift guard + track current dictionary release

The dictionary release moved to v1.2.1 while DICT_VERSION stayed pinned at
v1.0.0 in CI and the image/compose seed defaults. Two problems:

1. CI validated against a stale dictionary.
2. The contour seed could be bumped on a live volume, which silently relabels
   the already-seeded bytes — voiding games pinned to the prior label and
   serving the wrong dictionary for new ones. The flat DAWGs carry no embedded
   version, so this drift was undetectable.

Changes:

- Seed-drift guard: OpenWithVersions records the flat dir's version in a
  .seed_version marker on first boot and refuses to start when a later
  BACKEND_DICT_VERSION disagrees. DICT_VERSION is now the seed for a *fresh*
  volume only; a live contour migrates through the admin console (old versions
  stay resident, in-progress games keep replaying).
- Track the current release: CI's DICT_VERSION centralised to one workflow-level
  env (v1.2.1); image/compose/.env seed defaults bumped to v1.2.1. The deploy
  job keeps reading the per-contour vars.TEST_DICT_VERSION.
- Docs: ARCHITECTURE §5 (decision record), backend/deploy READMEs, PRERELEASE
  tracker (DV row).

Verified locally against the v1.2.1 artifact: gofmt, build, vet, unit and
integration (-tags=integration) all green.

docs/ARCHITECTURE.md

@@ -273,9 +273,18 @@ Key points:
   skipping the `.staging/` upload area) and restores the active pointer from
   `dictionary_state`. The volume preserves uploaded versions across redeploys;
   once seeded it is not re-seeded, so after bootstrap dictionary changes go through
-  the console rather than a rebuild. (The dictionaries ship as a versioned
-  **release artifact** from the `scrabble-dictionary` repo; the build's
-  `DICT_VERSION` selects only the seed.)
+  the console rather than a rebuild. Because the flat DAWGs carry no embedded
+  version, `OpenWithVersions` records the version the flat directory was first
+  opened at in a `.seed_version` marker on the volume and **refuses to boot** when a
+  later `BACKEND_DICT_VERSION` disagrees — the **seed-drift guard**: it stops a
+  bumped build seed on a live volume from relabelling the already-seeded bytes,
+  which would silently serve the wrong dictionary and void games pinned to the prior
+  label. A running contour therefore moves to a new release **through the console**
+  (the prior version stays resident, so its games keep replaying); `DICT_VERSION` is
+  the seed for a **fresh** volume only, set per contour from the deploy's
+  `TEST_`/`PROD_DICT_VERSION`. (The dictionaries ship as a versioned **release
+  artifact** from the `scrabble-dictionary` repo; the build's `DICT_VERSION` selects
+  only the seed.)
 - Move generation/validation/scoring use `Solver.GenerateMoves` (ranked),
   `Solver.ValidatePlay` and `Solver.ScorePlay`; board mutation uses
   `scrabble.Apply`. The engine adds its own deterministic, seeded tile **bag**