fix: quantise the light aperture amount of moving covers (#366)

A gate or door bound to a position-reporting cover fed current_position
into the light-barrier signature at toFixed(3) precision: every percent of
movement produced a new fingerprint, a full physicalBodyParts recompute
and a recut — up to ~100 heavy passes per gate cycle, plus LRU churn.

The light pipeline now consumes one quantised amount
(OPENING_LIGHT_AMOUNT_QUANTUM = 0.05, exact 0 and 1 nodes) at the single
point that feeds BOTH the signature and the cut geometry, so the cache key
and the drawn aperture agree by construction and a full sweep costs at
most 21 recomputes. The door LEAF animation stays smooth — _openingAmt is
quantised only for the light pipeline, nowhere else. Binary contact doors
are byte-identical to the previous behaviour (pinned by unit).

Assumption recorded in the spec: the 5% visual step of the light cut is
indistinguishable on real plans; if field impressions disagree, the
quantum is a one-constant change (0.02 => <=51 recomputes) or the decision
falls back to a debounce. LIGHT.md §Caching documents the grid.

Issue: #366
User-Visible: yes

scripts/mutation-gate.mjs

@@ -680,6 +680,18 @@ const MUTANT_DEFINITIONS = [
       replace: '    this._languageFailureUnsub = undefined;',
     }],
   },
+  {
+    id: 'opening-light-quantum-identity',
+    guard: 'node --test --test-name-pattern="#366" test/logic.test.mjs',
+    because: 'an identity quantum brings back ~100 barrier recomputes per moving-gate cycle — '
+      + 'the sweep unit must bound distinct signatures to the 0.05 grid (#366)',
+    patches: [{
+      file: 'src/logic.ts',
+      find: '  return Math.min(1, Math.max(0,\n'
+        + '    Math.round(safe / OPENING_LIGHT_AMOUNT_QUANTUM) * OPENING_LIGHT_AMOUNT_QUANTUM));',
+      replace: '  return safe;',
+    }],
+  },
   {
     id: 'backdrop-probe-always-safe',
     guard: 'node demo/smoke_backdrop_guard.mjs',