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.

backend/internal/engine/registry.go

@@ -71,13 +71,21 @@ func Open(dir, version string, variants ...Variant) (*Registry, error) {
 // 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). A partially
-// loaded registry is closed before any error is returned.
+// 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
+// registry is closed before any error is returned.
 func OpenWithVersions(dir, bootVersion string) (*Registry, error) {
 	r, err := Open(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()
+		return nil, err
+	}
 	entries, err := os.ReadDir(dir)
 	if err != nil {
 		_ = r.Close()

backend/internal/engine/reload_test.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -112,6 +113,59 @@ func TestOpenWithVersionsSkipsDotDirs(t *testing.T) {
 	}
 }
 
+// TestOpenWithVersionsRecordsSeedMarker verifies the first boot records the seed
+// version in the flat dir's marker, the marker is not mistaken for a version, and a
+// reboot at the same seed version succeeds.
+func TestOpenWithVersionsRecordsSeedMarker(t *testing.T) {
+	dir := t.TempDir()
+	for _, v := range Variants() {
+		copyDawg(t, testDictDir(), dir, v)
+	}
+
+	reg, err := OpenWithVersions(dir, "v1")
+	if err != nil {
+		t.Fatalf("first open: %v", err)
+	}
+	if got := reg.Versions(VariantEnglish); len(got) != 1 || got[0] != "v1" {
+		t.Errorf("versions = %v, want only [v1] (marker not a version)", got)
+	}
+	_ = reg.Close()
+
+	data, err := os.ReadFile(filepath.Join(dir, seedMarkerFile))
+	if err != nil {
+		t.Fatalf("read seed marker: %v", err)
+	}
+	if got := strings.TrimSpace(string(data)); got != "v1" {
+		t.Fatalf("seed marker = %q, want v1", got)
+	}
+
+	reg2, err := OpenWithVersions(dir, "v1")
+	if err != nil {
+		t.Fatalf("reboot at same seed: %v", err)
+	}
+	_ = reg2.Close()
+}
+
+// TestOpenWithVersionsRejectsSeedDrift verifies the guard refuses to boot a directory
+// seeded as one version when BACKEND_DICT_VERSION (bootVersion) names another — the
+// seed-drift footgun a bumped build seed on a live volume would cause.
+func TestOpenWithVersionsRejectsSeedDrift(t *testing.T) {
+	dir := t.TempDir()
+	for _, v := range Variants() {
+		copyDawg(t, testDictDir(), dir, v)
+	}
+
+	reg, err := OpenWithVersions(dir, "v1")
+	if err != nil {
+		t.Fatalf("seed open: %v", err)
+	}
+	_ = reg.Close()
+
+	if _, err := OpenWithVersions(dir, "v2"); err == nil {
+		t.Fatal("open after seed bump: want error, got nil")
+	}
+}
+
 // TestReloadRegistersNewVersion verifies Load adds a second version to a variant
 // already resident, moves the latest pointer and keeps the earlier version.
 func TestReloadRegistersNewVersion(t *testing.T) {

backend/internal/engine/seedmarker.go

@@ -0,0 +1,52 @@
+package engine
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// seedMarkerFile names the file, in the flat dictionary directory, that records the
+// version the directory was first seeded as. It is dot-prefixed so OpenWithVersions'
+// version scan skips it (like the .staging upload area).
+const seedMarkerFile = ".seed_version"
+
+// checkSeedMarker reconciles the flat dictionary directory's recorded seed version
+// with bootVersion, guarding the seed-drift footgun.
+//
+// The contour's dictionary lives on a named volume seeded from the image once and
+// never re-seeded (deploy/docker-compose.yml). The flat directory's DAWG files carry
+// no embedded version, so their version is only the label BACKEND_DICT_VERSION gives
+// them. Bumping the build seed on a live volume would therefore relabel the already
+// seeded bytes: games that pinned the old label become unreplayable (voided) and new
+// games would silently use the wrong dictionary. checkSeedMarker records the seed
+// version on a fresh directory and, on every later boot, returns an error when
+// bootVersion no longer matches the recorded seed — so an operator changes a live
+// dictionary by uploading the new release through the admin console, or wipes the
+// volume to re-seed, never by bumping the build seed (docs/ARCHITECTURE.md §5).
+//
+// A directory that cannot be written makes the first record fail; that already breaks
+// the admin console (which writes version subdirectories here), so the error is
+// returned rather than swallowed, matching the package's fail-loud dictionary setup.
+func checkSeedMarker(dir, bootVersion string) error {
+	path := filepath.Join(dir, seedMarkerFile)
+	data, err := os.ReadFile(path)
+	switch {
+	case err == nil:
+		if recorded := strings.TrimSpace(string(data)); recorded != bootVersion {
+			return fmt.Errorf("engine: dictionary volume was seeded as %q but BACKEND_DICT_VERSION is %q; "+
+				"change a live dictionary by uploading the release through the admin console, or wipe the "+
+				"volume to re-seed — do not bump the build seed (docs/ARCHITECTURE.md §5)", recorded, bootVersion)
+		}
+		return nil
+	case errors.Is(err, os.ErrNotExist):
+		if err := os.WriteFile(path, []byte(bootVersion+"\n"), 0o644); err != nil {
+			return fmt.Errorf("engine: record dictionary seed marker %s: %w", path, err)
+		}
+		return nil
+	default:
+		return fmt.Errorf("engine: read dictionary seed marker %s: %w", path, err)
+	}
+}