feat: catalog-level validate_catalog + index_by_id (unique ids)

Per sub-project-2 plan Task 2 / spec §3. Purely additive to hef.catalog;
no existing symbol changes. validate_catalog runs per-record validate() then
asserts id uniqueness; index_by_id gives by-id lookup the player also wants.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hef/catalog.py

@@ -126,3 +126,20 @@ def append_record(record, path) -> None:
     path = Path(path)
     with path.open("a", encoding="utf-8") as fh:
         fh.write(json.dumps(record_to_dict(record), ensure_ascii=False) + "\n")
+
+
+def index_by_id(records) -> dict:
+    """Map id -> Record, raising CatalogError on a duplicate id."""
+    idx: dict = {}
+    for r in records:
+        if r.id in idx:
+            raise CatalogError(f"duplicate record id: {r.id!r}")
+        idx[r.id] = r
+    return idx
+
+
+def validate_catalog(records) -> None:
+    """Validate every record AND cross-record invariants (currently: unique ids)."""
+    for r in records:
+        validate(r)
+    index_by_id(records)  # raises on duplicate id