feat(topdata): parts autogen from CDN part.yml channel

Phase 2 of the parts-row autogen design: Crucible resolves parts 2DA rows
from the immutable part.yml published with the asset HAK release, over the
anonymous Bunny CDN channel, with the parts consumer required (no fail-open).

- generalize resolveCDNChannelManifest: derive manifest basename + provenance
  label from config; drop hardcoded assets/vfxs.yml so part.yml resolves too
- add filterPartsCDNChannelEntries + mode dispatch implementing the inventory
  contract (restype mdl under part/<cat>/, trailing-digit row id, dedup
  l/r/race/gender variants, sort by source, zero accepted rows = hard fail)
- support hand category + parts/hand dataset mapping (12 datasets total)
- native pipeline: single augment -> normalize -> override sequence with the
  configured parts_rows policy; overrides may update but never synthesize a row
- reject multiple parts_rows consumers in project validation
- tests: parts filter contract, category coverage, offline basename, orphan
  override reject, and a parity guard proving a bare CDN-only parts build

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/topdata/parts_discovery.go

@@ -21,6 +21,7 @@ var supportedPartCategories = []string{
 	"chest",
 	"foot",
 	"forearm",
+	"hand",
 	"leg",
 	"neck",
 	"pelvis",
@@ -35,6 +36,7 @@ var partDatasetToAssetCategory = map[string]string{
 	"chest":    "chest",
 	"foot":     "foot",
 	"forearm":  "forearm",
+	"hand":     "hand",
 	"legs":     "leg",
 	"neck":     "neck",
 	"pelvis":   "pelvis",
@@ -390,6 +392,18 @@ func applyPartOverrides(sourceDir string, collected []nativeCollectedDataset) ([
 	return applyPartOverridesWithConfig(sourceDir, collected, project.PartsRowsConfig{})
 }
 
+// partsRowsConfigForProject returns the PartsRows policy of the single configured
+// parts_rows autogen consumer, or the zero value if none. Project validation
+// guarantees at most one parts_rows consumer, so the first match is canonical.
+func partsRowsConfigForProject(p *project.Project) project.PartsRowsConfig {
+	for _, c := range p.Config.Autogen.Consumers {
+		if strings.TrimSpace(c.Mode) == "parts_rows" {
+			return c.PartsRows
+		}
+	}
+	return project.PartsRowsConfig{}
+}
+
 func applyPartOverridesWithConfig(sourceDir string, collected []nativeCollectedDataset, cfg project.PartsRowsConfig) ([]nativeCollectedDataset, error) {
 	result := make([]nativeCollectedDataset, len(collected))
 	copy(result, collected)
@@ -430,9 +444,7 @@ func applyPartOverridesWithConfig(sourceDir string, collected []nativeCollectedD
 			}
 			row, ok := rowByID[rowID]
 			if !ok {
-				row = createDefaultPartRowForDataset(rowID, dataset.Dataset.Name, dataset.Columns, cfg)
-				rows = append(rows, row)
-				rowByID[rowID] = row
+				return nil, fmt.Errorf("parts/%s override %d targets row id %d which is neither a baseline row nor a discovered model row; author the row in data/parts/%s.json instead", category, index, rowID, category)
 			}
 			for field, value := range override {
 				if field == "id" {