perf: the clean-floor cache holds one entry per room per config epoch (#509)

The pre-release perf gate of the v1.74.0-beta.1 candidate reported
cache.entries.cleanFloor 120 against a ceiling of 100 (run 34480302982,
large-house-interaction-v1). The ceiling was calibrated when only the visible
space populated `_cleanFloorCache`; since #509 the summary panel computes the
clean-floor total in per-room slices through the same cache, so the sixty
fixture rooms are cached under both config epochs the interaction profile
creates — 60 × 2 = 120, exactly what the run measured.

The ceiling moves to 180 in the smoke and in the full interaction profile:
one entry per room per epoch with room for a third epoch, far below the LRU
cap of 600 and far below anything a per-frame or per-marker regression would
produce. The leak detector is untouched: cacheGrowth.cleanFloor stays 0.

Issue: #509
User-Visible: no
Release: v1.74.0-beta.1

demo/performance/budgets-interaction-smoke.json

@@ -96,7 +96,7 @@
     "hardMaxGrowthBytes": 67108864
   },
   "cacheEntries": {
-    "cleanFloor": 100,
+    "cleanFloor": 180,
     "glowClip": 200,
     "wallUnion": 1,
     "openingTunnel": 1,

demo/performance/budgets-large-house-interaction.json

@@ -118,7 +118,7 @@
     "noiseAllowanceBytes": 16777216
   },
   "cacheEntries": {
-    "cleanFloor": 100,
+    "cleanFloor": 180,
     "glowClip": 200,
     "wallUnion": 1,
     "openingTunnel": 1,