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/internal/engine/registry.go

@@ -70,20 +70,22 @@ func Open(dir, version string, variants ...Variant) (*Registry, error) {
 // immediate subdirectory of dir: a subdirectory named V contributes, under
 // version V, the variants whose committed DAWG it carries. This is the
 // restart-side of the admin dictionary reload — a version reloaded into dir/<V>/
-// at runtime is resident again after a restart. A subdirectory named like the
-// boot version is skipped (the flat dir already is the boot version). It records
-// and enforces a seed-version marker on the flat dir (see checkSeedMarker), failing
-// when the build seed was bumped on an already-seeded volume. A partially loaded
+// at runtime is resident again after a restart. The flat dir's version is resolved
+// from its .seed_version marker (see resolveSeedVersion): a fresh dir records
+// bootVersion, an already-seeded dir keeps its recorded label and ignores bootVersion,
+// so a bumped build seed never relabels live bytes. A subdirectory named like the
+// resolved seed version is skipped (the flat dir already is it). A partially loaded
 // registry is closed before any error is returned.
 func OpenWithVersions(dir, bootVersion string) (*Registry, error) {
-	r, err := Open(dir, bootVersion)
+	// Resolve the flat dir's version from its seed marker first: on an already-seeded
+	// volume the marker wins and bootVersion is ignored, so a bumped build seed cannot
+	// relabel live bytes (see resolveSeedVersion).
+	seed, err := resolveSeedVersion(dir, bootVersion)
 	if err != nil {
 		return nil, err
 	}
-	// Guard the seed-drift footgun before scanning for additional versions: refuse
-	// to boot when the flat dir was seeded under a different version than bootVersion.
-	if err := checkSeedMarker(dir, bootVersion); err != nil {
-		_ = r.Close()
+	r, err := Open(dir, seed)
+	if err != nil {
 		return nil, err
 	}
 	entries, err := os.ReadDir(dir)
@@ -92,9 +94,9 @@ func OpenWithVersions(dir, bootVersion string) (*Registry, error) {
 		return nil, fmt.Errorf("engine: scan dictionary dir %s: %w", dir, err)
 	}
 	for _, e := range entries {
-		// Skip non-directories, the boot version (already loaded as the flat dir)
-		// and dot-prefixed directories (the upload staging area, dir/.staging/).
-		if !e.IsDir() || e.Name() == bootVersion || strings.HasPrefix(e.Name(), ".") {
+		// Skip non-directories, the resolved seed version (already loaded as the flat
+		// dir) and dot-prefixed directories (the upload staging area, dir/.staging/).
+		if !e.IsDir() || e.Name() == seed || strings.HasPrefix(e.Name(), ".") {
 			continue
 		}
 		if _, err := r.LoadAvailable(filepath.Join(dir, e.Name()), e.Name()); err != nil {