fix(nwsync): pin emitter version, ship the shim, harden manifest reads

Review follow-ups on the same work:

- emitter_version is its own sidecar field, not the build revision. Keying
  the merge check on created_with invalidated every published index on every
  unrelated crucible commit, forcing a re-emit of the whole corpus.
- flake.nix builds cmd/crucible-nwsync, so Nix consumers get the binary the
  docs promise.
- readManifest uses io.ReadFull: bytes.Reader.Read can return a short count
  with no error, so a truncated manifest parsed into zero-padded garbage.
- emit refuses a .mod outright and names why, rather than failing on an
  unknown extension.
- SOURCE_DATE_EPOCH pins the sidecar timestamp; the manifest was already
  deterministic.
- Entry.identity replaces the resref/restype key struct duplicated in emit
  and assemble.

Refs #53.

internal/nwsync/nwsync_test.go

@@ -361,7 +361,7 @@ func TestAssembleRefusesMismatchedEmitterVersions(t *testing.T) {
 	if err := json.Unmarshal(body, &sidecar); err != nil {
 		t.Fatal(err)
 	}
-	sidecar.CreatedWith = "crucible some-other-build"
+	sidecar.EmitterVersion = "0"
 	patched, err := marshalSidecar(sidecar)
 	if err != nil {
 		t.Fatal(err)
@@ -380,6 +380,45 @@ func TestAssembleRefusesMismatchedEmitterVersions(t *testing.T) {
 	}
 }
 
+func TestEmitHonoursSourceDateEpoch(t *testing.T) {
+	t.Setenv("SOURCE_DATE_EPOCH", "1700000000")
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "pinned.hak")
+	writeHak(t, hak, map[string][]byte{"one1.tga": []byte("body")})
+	result, err := Emit(hak, filepath.Join(dir, "out"))
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+	body, err := os.ReadFile(result.ManifestPath + ".json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var sidecar Sidecar
+	if err := json.Unmarshal(body, &sidecar); err != nil {
+		t.Fatal(err)
+	}
+	if sidecar.Created != 1700000000 {
+		t.Errorf("created = %d, want the pinned SOURCE_DATE_EPOCH", sidecar.Created)
+	}
+}
+
+func TestEmitRejectsAModule(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sow.mod")
+	var out bytes.Buffer
+	if err := erf.Write(&out, erf.New("MOD", []erf.Resource{
+		{Name: "module", Type: restype(t, "ifo"), Data: []byte("ifo"), Size: 3},
+	})); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, out.Bytes(), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Emit(path, filepath.Join(dir, "out")); err == nil {
+		t.Fatal("emit accepted a .mod; a manifest never carries module contents")
+	}
+}
+
 func TestAssembleFailsClosedOnMissingIndex(t *testing.T) {
 	entriesDir, _, _ := emitFixture(t)
 	_, err := Assemble(AssembleOptions{