perf(floor): key the floor-geometry caches by the floor's content (#744)

The physical bodies, the wall union pool, the inner room contours and the
clean floor carried the global config epoch in their keys. Every edit of
any floor bumps it, so after one edit every other floor was cold again:
in large-house the first visit to an untouched floor rebuilt its wall
union and paid ~0.7 s flat / ~0.65 s 2.5D instead of ~40-55 ms.

A floor's geometry reads only its own config record (spaceModels) and
constants, so the key is now a content fingerprint of that record
(src/floor-geometry-key.ts), remembered per epoch and per record object.
The geometry also reads the current floor's config next to the model it
is given; when those records differ the key covers both. The live resize
preview is its own record, so preview frames get their own key; the
editor runtime seeds the pool and re-keys the bodies through the same
reader. The stairs editor no longer clears the clean floors of every
floor: the stairs are part of the floor's record.

The #735 switch-cycle guard now also sees the union pool and the inner
contours (optional members of the large-house card contract, so an older
comparison bundle reads 0). smoke_floor_geometry_cache proves the warm other floor and the
invalidation against an independent card (multi-floor push with shared
walls, a stair, a resize preview and its cancel); two mutants guard it.

Issue: #744
User-Visible: yes
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

docs/testing-notes/mutation-browser-guards.md

@@ -17,8 +17,8 @@ to prove that the Node witness actually kills it.
 | Paint, cascade and layer composition | 26 | The invariant depends on computed CSS, SVG paint, clipping, stacking or pixels produced by Chromium. |
 | Pointer geometry and trusted interaction | 45 | The invariant depends on hit testing, pointer capture, touch/keyboard dispatch or live DOM geometry. |
 | Responsive DOM layout | 38 | The invariant depends on measured element boxes, responsive breakpoints, native/HA dialog shells or focusable target size. |
-| Custom-element and HA browser lifecycle | 97 | The invariant crosses Lit/custom-element lifecycle, browser storage/events, lazy loading or a complete HA-card state transition. |
-| **Total** | **214 / 200** | Above the guideline `mutation-gate --check` warns rather than fails (#699); each guard above it is held by its own reason in this inventory and its `because`. |
+| Custom-element and HA browser lifecycle | 99 | The invariant crosses Lit/custom-element lifecycle, browser storage/events, lazy loading or a complete HA-card state transition. |
+| **Total** | **216 / 200** | Above the guideline `mutation-gate --check` warns rather than fails (#699); each guard above it is held by its own reason in this inventory and its `because`. |
 
 ## Measured effect
 
@@ -237,6 +237,8 @@ The invariant crosses Lit/custom-element lifecycle, browser storage/events, lazy
 - `editor-neutral-escape-does-not-exit`
 - `empty-space-cleanup-disabled`
 - `fixed-floor-transition-guard-bypassed`
+- `floor-geometry-key-global-epoch`
+- `floor-geometry-key-ignores-content`
 - `french-locale-wrong-dictionary`
 - `hp-dialog-escape-does-not-close`
 - `junction-limit-baseline-cache-stale`