fix(performance): report slack usage only for entries that have slack

A wall-clock measurement above `value` but within `value × toleranceRatio`
fell into the slack branch and was reported as using "slack", conflating
timing tolerance with counter slack. Only entries with `slack` report it now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0166PWobUjpoiWt8E9bBdsCs

tools/performance/check-journey-ratchet.mjs

@@ -188,7 +188,7 @@ export function compareToBaselines({ baselines, summary, only = [] }) {
                 result.failures.push(
                     `${label}: ${formatNumber(measured)}${unit} exceeds ${limitText} by ${formatNumber(measured - limit)}${unit}. Bring the value back down; baselines only move down. If the growth is a deliberate trade-off, raise the baseline in ${DEFAULT_BASELINES_PATH}, make the case in the PR, and ask a maintainer to add the ${BASELINE_INCREASE_LABEL} label.`
                 );
-            } else if (measured > entry.value) {
+            } else if (entry.slack && measured > entry.value) {
                 result.passed.push(
                     `${label}: ${formatNumber(measured)}${unit} within ${limitText}; uses ${formatNumber(measured - entry.value)} of ${formatNumber(limit - entry.value)}${unit} slack.`
                 );

tools/performance/check-journey-ratchet.test.mjs

@@ -155,8 +155,9 @@ test('wall-clock entries fail only above value × toleranceRatio', () => {
     assert.deepEqual(within.failures, []);
     assert.match(
         within.passed[1],
-        /1,250 ms within 1,250 \(baseline 1,000 × 1\.25\)/
+        /1,250 ms within 1,250 \(baseline 1,000 × 1\.25\)\.$/
     );
+    assert.doesNotMatch(within.passed[1], /slack/);
 
     const above = compareToBaselines({
         baselines,