perf: accept the lazy isometric chunk boot-task split in the isometric long-task count budget (#507)

Full Performance of the v1.73.0 stable candidate against the v1.72.0 product
(run 34354409872) was red on one check of the isometric profile:
longTask.countP95 16 → 20 against max(16×1.2, 16+3) = 19.2, with every
timing, longTask.totalP95Ms and longTask.maxSingleMs green. The trace behind
#506 shows why: since v1.73.0-beta.1 the isometric renderer is the lazy
iso-scene-render chunk (#160 Stage 3), so the single v1.72.0 boot task is
split in two around that import — the same work, +2 tasks.

Owner decision 2026-09-09: accept the split. countNoiseAllowance 3 → 5 for
large-house-isometric-v1 only; the ratio, the hard ceiling, total and
maximum single task keep gating real growth. The downloaded CI artefact
re-evaluated with this budget passes (limit 21, actual 20, no failures).
Documented in demo/performance/README.md; the budget test pins the
allowance and the untouched profiles.

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

demo/performance/README.md

@@ -218,6 +218,17 @@ 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
+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
+`longTask.maxSingleMs` keep their unchanged ratios and still catch real
+growth — but `longTask.countP95` counts the split as +2 tasks and, with
+runner jitter, sat at 16 → 20 against a 19.2 limit on the v1.73.0 stable
+comparison. The allowance widens the count check alone to 16 → 21 for that
+baseline; it is not a licence for more work per task.
+
 The `cleanFloor` entry ceiling is 100: the reviewed fixture warms exactly 100
 deterministic room/physical-body entries. An extra 20 means that one complete
 floor was invalidated and rebuilt, so fixture extensions must recalibrate this

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

@@ -19,7 +19,7 @@
     "maxSingleNoiseAllowanceMs": 250,
     "maxCountP95": 30,
     "maxCountRegressionRatio": 0.2,
-    "countNoiseAllowance": 3,
+    "countNoiseAllowance": 5,
     "maxTotalP95Ms": 12000,
     "maxTotalRegressionRatio": 0.2,
     "noiseAllowanceMs": 150

test/performance-budget.test.mjs

@@ -256,3 +256,18 @@ test('interaction aggregate keeps hosted-runner headroom without weakening compo
   assert.equal(smoke.timings.cameraSeriesMs.hardMaxMs, 500);
   assert.equal(smoke.timings.editorSeriesMs.hardMaxMs, 750);
 });
+
+test('isometric long-task count allowance covers the lazy iso-chunk split, nothing else (#507)', () => {
+  const isometric = readBudget('budgets-large-house-isometric.json');
+  assert.equal(isometric.longTasks.countNoiseAllowance, 5, 'owner-accepted +2 tasks of the lazy iso-scene-render split plus jitter');
+  assert.equal(isometric.longTasks.maxCountRegressionRatio, 0.2, 'the ratio is not the lever');
+  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);
+  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`);
+  }
+  // The v1.73.0 stable comparison against v1.72.0: 16 → 20 passes, 22 does not.
+  const limit = Math.max(16 * (1 + isometric.longTasks.maxCountRegressionRatio), 16 + isometric.longTasks.countNoiseAllowance);
+  assert.equal(limit, 21);
+});