Document GC 4.75 catalog trailer

docs/re_gc_catalog.md

@@ -12,6 +12,12 @@ record count, allocates `count * 0xb4` bytes, and deserializes every variable-si
 disk record into one `0xb4`-byte runtime entry. The dumped file contains 924
 records. IDs are explicit and can have gaps; they are not array indices.
 
+The loader stops after the declared record count; it does not require EOF.
+GC 4.75 catalogs can append a version-specific metadata block after the song
+array (one observed 917-record catalog has 4116 trailing bytes). The clean-room
+reader therefore validates every declared record but intentionally leaves such
+post-array data uninterpreted.
+
 The game's primitives used by this loader are:
 
 - `FUN_005d8bc0`: big-endian `u32`;