refactor: normalize each system id once

SelectionIndex.build and system_group called _norm_system_id 1.7 million times per manifest run for about a thousand ids, each through the manufacturer prefix loop. The function reads only its argument and two constant tables, so its answer is cached. Measured on the call volume of one manifest run (630 ids, 1,686,618 calls): 1.53 s uncached, 0.07 s cached. The end-to-end manifest run moves inside its own noise on this host (45.9 to 50.6 s across four runs).

scripts/common.py

@@ -3036,12 +3036,16 @@ SYSTEM_ALIASES: dict[str, str] = {
 }
 
 
+@functools.lru_cache(maxsize=None)
 def _norm_system_id(sid: str) -> str:
     """Normalize system ID for cross-platform matching.
 
     Resolves known aliases, then strips manufacturer prefixes and separators
     so that platform-specific IDs (e.g., "xbox", "nintendo-wiiu") match
     profile IDs (e.g., "microsoft-xbox", "nintendo-wii-u").
+
+    A function of the string and two constant tables: a manifest run asks it
+    1.7 million times for about 1,100 ids.
     """
     s = sid.lower().replace("_", "-")
     s = SYSTEM_ALIASES.get(s, s)