fix(engine): make .seed_version marker authoritative (no boot refusal)

The seed-drift guard shipped as refuse-boot: the backend exited when
BACKEND_DICT_VERSION disagreed with the flat dir's recorded .seed_version. On
the test contour that turned a harmless-in-intent action — bumping the
TEST_DICT_VERSION variable to the active release (v1.2.1) on a volume seeded as
v1.0.0 — into a crash loop, because DICT_VERSION is the *seed* of a fresh
volume, not the active version (which the admin console drives).

Make the marker authoritative instead: OpenWithVersions resolves the flat dir's
version from .seed_version when present and ignores bootVersion on an
already-seeded volume; bootVersion only seeds a fresh volume's marker. So a
bumped build seed on a live volume is a no-op (it can't relabel live bytes and
can't void games pinned to the prior label), and it correctly seeds the next
fresh volume. The subdirectory scan now skips the resolved seed, so a version
also present as a subdir (e.g. v1.2.1 uploaded via the console while the build
seed is bumped to v1.2.1) is still loaded rather than shadowed by the flat bytes.

Tests: marker-wins over a bumped boot version; a bumped boot keeps the matching
subdir resident (the live-contour case). Docs updated (ARCHITECTURE §5, READMEs,
compose/.env, PRERELEASE DV) from "refuses to boot" to "marker wins / ignored".

Verified locally against v1.2.1: gofmt, build, vet, unit, integration green.

docs/ARCHITECTURE.md

@@ -274,16 +274,18 @@ Key points:
   `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. 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
+  version, `OpenWithVersions` records the version the flat directory was first seeded
+  at in a `.seed_version` marker on the volume and treats that marker as
+  **authoritative** (the **seed-drift guard**): on an already-seeded volume a later
+  `BACKEND_DICT_VERSION` is ignored, so a bumped build seed cannot relabel the
+  already-seeded bytes — which would otherwise 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), and `DICT_VERSION` is the seed for a **fresh** volume only:
+  bumping it on a live contour is a harmless no-op that takes effect on the next
+  fresh volume. Set it 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