perf: give the Stage 3 dense isometric budget the same long-task count allowance (#507)

The #160 contract test keeps the dense profile's longTasks block equal to
the historical isometric one, and both profiles boot through the same lazy
iso-scene-render chunk; the accepted split applies to both. Validate
34356856702 caught the divergence.

Issue: #507
User-Visible: no
Release: v1.73.0

demo/performance/README.md

@@ -218,8 +218,8 @@ rationale in the change. Do not loosen a threshold merely to make a single red
 run pass. A new fixture profile gets a new profile id instead of silently
 changing the meaning of `large-house-v1`.
 
-`large-house-isometric-v1` carries `countNoiseAllowance: 5` (the other
-profiles keep 3). Owner decision 2026-09-09, #507: since v1.73.0-beta.1 the
+`large-house-isometric-v1` and its Stage 3 dense twin carry
+`countNoiseAllowance: 5` (the other profiles keep 3). Owner decision 2026-09-09, #507: since v1.73.0-beta.1 the
 isometric renderer lives in the lazy `iso-scene-render` chunk (#160 Stage 3),
 so the single v1.72.0 boot task is split into two around that import. The
 load-phase work is unchanged — timings, `longTask.totalP95Ms` and

demo/performance/budgets-isometric-stage3-dense.json

@@ -21,7 +21,7 @@
     "maxSingleNoiseAllowanceMs": 250,
     "maxCountP95": 30,
     "maxCountRegressionRatio": 0.2,
-    "countNoiseAllowance": 3,
+    "countNoiseAllowance": 5,
     "maxTotalP95Ms": 12000,
     "maxTotalRegressionRatio": 0.2,
     "noiseAllowanceMs": 150

test/performance-budget.test.mjs

@@ -264,6 +264,7 @@ test('isometric long-task count allowance covers the lazy iso-chunk split, nothi
   assert.equal(isometric.longTasks.maxCountP95, 30);
   assert.equal(isometric.longTasks.maxTotalRegressionRatio, 0.2, 'total work is still gated as before');
   assert.equal(isometric.longTasks.maxSingleRegressionRatio, 0.2);
+  assert.equal(readBudget('budgets-isometric-stage3-dense.json').longTasks.countNoiseAllowance, 5, 'the dense twin shares the lazy iso chunk and the #160 common-ceiling contract');
   for (const file of ['budgets.json', 'budgets-large-house-plan-snap.json', 'budgets-large-house-interaction.json']) {
     assert.equal(readBudget(file).longTasks.countNoiseAllowance, 3, `${file} keeps the ordinary allowance`);
   }