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.

backend/README.md

@@ -190,7 +190,7 @@ internal/ratewatch/  # gateway rate-limit reports: episode window for the consol
 | `BACKEND_OTEL_TRACES_EXPORTER` | `none` | `none`, `stdout` or `otlp` (gRPC; endpoint from the standard `OTEL_EXPORTER_OTLP_*`). |
 | `BACKEND_OTEL_METRICS_EXPORTER` | `none` | `none`, `stdout` or `otlp`. |
 | `BACKEND_DICT_DIR` | — | **Required.** Directory of committed `.dawg` dictionaries. |
-| `BACKEND_DICT_VERSION` | `v1` | Seed dictionary version new games pin (the flat dir's label). Recorded in a `.seed_version` marker on first boot; the backend refuses to start if a later value drifts from it — the seed-drift guard (ARCHITECTURE.md §5). |
+| `BACKEND_DICT_VERSION` | `v1` | Version label for the flat dictionary dir. Recorded in a `.seed_version` marker on first boot and authoritative after: on a seeded volume a changed value is ignored (it seeds only a fresh volume) — the seed-drift guard (ARCHITECTURE.md §5). |
 | `BACKEND_GAME_TIMEOUT_SWEEP_INTERVAL` | `1m` | How often the turn-timeout sweeper runs. |
 | `BACKEND_GAME_CACHE_TTL` | `24h` | Idle window before a live game is evicted from cache. |
 | `BACKEND_LOBBY_ROBOT_WAIT` | `10s` | Auto-match wait before a robot is substituted for a missing human. |
@@ -255,9 +255,10 @@ repo (one semver per set); the engine loads them by `(variant, dict_version)` fr
 labelled `BACKEND_DICT_VERSION`; uploaded versions live in `<version>/`
 subdirectories the admin console writes and a restart re-loads. Because the DAWGs
 carry no embedded version, the first boot records the seed in a `.seed_version`
-marker and a later boot **refuses to start** if `BACKEND_DICT_VERSION` no longer
-matches it (the seed-drift guard), so a live contour's dictionary is changed through
-the console, never by bumping the build seed (ARCHITECTURE.md §5).
+marker that is authoritative after: on a seeded volume a changed `BACKEND_DICT_VERSION`
+is ignored (it seeds only a fresh volume) — the seed-drift guard — so a live contour's
+dictionary is changed through the console, never by bumping the build seed
+(ARCHITECTURE.md §5).
 
 ## Tests