fix: the baseline cache keys on geometry, and the smoke tells all three worlds apart (#330 r2-M1)

The reviewer proved my behavioural claim false by running the stale-cache
mutant against the smoke: 11 vs 12 total calls — indistinguishable. Two real
defects hid behind that finding:

1. The cache keyed on _cfgEpoch, which ticks on every ACCEPTED PREVIEW —
   the cache missed on every pointermove and the baseline was recomputed
   ~4 times per gesture (measured). The key is now the document identity
   plus spacePhysicalGeometryFingerprint of its space: content, not a
   counter. A preview overlay leaves the fingerprint alone; an in-place
   structural commit changes it and honestly invalidates.

2. The smoke now counts BASELINE computations only (calls whose document is
   _serverCfg) across two gestures with a commit in between, expecting
   exactly 1 then exactly 2. A disabled cache lands near 20, an eternal
   cache stays at 1 — every mutant class turns the smoke red, and the smoke
   is now the mutant's guard alongside the source-contract unit.

Issue: #330
User-Visible: no

scripts/mutation-gate.mjs

@@ -563,13 +563,18 @@ export const MUTANTS = [
   },
   {
     id: 'junction-limit-baseline-cache-stale',
-    guard: 'npx tsc -p tsconfig.test.json && node scripts/fix-test-build.mjs '
+    // r2-M1: the behavioural guard — the smoke counts BASELINE computations
+    // (exactly one per gesture with a working cache, one per pointermove
+    // without) — plus the source-contract unit stays as the cheap first line.
+    guard: 'node demo/smoke_junction_limits.mjs '
+      + '&& npx tsc -p tsconfig.test.json && node scripts/fix-test-build.mjs '
       + '&& node --test --test-name-pattern="#330 AC4" test/junction-limits.test.mjs',
     because: 'a baseline cache that survives a config epoch change serves verdicts of a '
       + 'plan that no longer exists — the epoch check IS the invalidation contract (#330 §4.4)',
     patches: [{
       file: 'src/houseplan-card.ts',
-      find: "    if (cached && cached.epoch === this._cfgEpoch && cached.spaceId === spaceId) {",
+      find: "    if (cached && fingerprint && cached.fingerprint === fingerprint\n"
+        + '        && cached.spaceId === spaceId) {',
       replace: '    if (cached && cached.spaceId === spaceId) {',
     }],
   },