fix: state one catalog ratio, correct the faq

The home page and the stats export counted every file carrying a
provenance record, the provenance page and the README only the system
files. The site published 553 and 566 for the same quantity, one click
apart, and the export paired the wider count with composition.systems as
its denominator. common.count_catalog_matched is now the single source,
scoped to the systems bucket.

The FAQ had drifted from the profiles it describes: MAME pinned at 0.287
against 0.289 in mame.yml, Adler-32 attributed to Dolphin's IPL rather
than the DSP ROMs that carry known_hash_adler32, and the per-emulator
verbose report named as the only content check on an existence platform,
which skips the DISCREPANCY line the platform report raises itself.

Tests read both sides: no generator may count matches inline, and each
FAQ claim is checked against the profile or the script that owns it.

scripts/generate_readme.py

@@ -18,8 +18,8 @@ from datetime import datetime, timezone
 
 sys.path.insert(0, os.path.dirname(__file__))
 from common import (
-    GAME_DATA_TOPS,
     compute_composition,
+    count_catalog_matched,
     list_registered_platforms,
     load_database,
     load_emulator_profiles,
@@ -184,25 +184,13 @@ _CATALOG_LABELS = {"redump": "Redump", "no-intro": "No-Intro", "tosec": "TOSEC"}
 
 
 def _catalog_matched_line(db: dict) -> list[str]:
-    """Bullet line for files matched to dump-preservation catalogs.
-
-    Counted against the system files alone: those catalogs index console and
-    computer dumps, so arcade ROM sets and engine data sit outside their
-    scope and would only dilute the ratio.
-    """
-    sources: set[str] = set()
-    matched = 0
-    for entry in db.get("files", {}).values():
-        provenance = entry.get("provenance")
-        if not provenance:
-            continue
-        sources.update(provenance)
-        parts = entry.get("path", "").split("/")
-        top = parts[1] if len(parts) > 1 else ""
-        if top != "Arcade" and top not in GAME_DATA_TOPS:
-            matched += 1
+    """Bullet line for files matched to dump-preservation catalogs."""
+    matched = count_catalog_matched(db)
     if not matched:
         return []
+    sources: set[str] = set()
+    for entry in db.get("files", {}).values():
+        sources.update(entry.get("provenance") or ())
     system_files = compute_composition(db)["systems"]["files"]
     labels = ", ".join(_CATALOG_LABELS.get(s, s) for s in sorted(sources))
     return [