docs(perf): record the runner's settled layout-shift measurement

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/performance-journeys.md

@@ -179,6 +179,12 @@ down. Something 316 px tall above it is removed and inserted again during
 startup, a flicker #1738 did not cover. The counter is working as intended;
 the flicker is a separate fix.
 
+On the Linux CI runner (`Performance journeys` job of #1756, run
+36618062068) the same flicker is a race: the measured iterations read
+`[0, 0, 0.235, 0, 0]` (`stable: false`, every window `quiet` about 540 ms
+after the first card), and the one hit shows the same two 316 px moves of
+the recent-sources rail.
+
 #### Main-process counters
 
 With `IPTVNATOR_PERF_CAPTURE=1`, which the journey sets,
@@ -650,10 +656,10 @@ in all eighteen runner iterations; the `spawnToFirstCardMs` P50 ranged from
 1,401 to 1,674 ms. All four stay evidence for now. Runner counters also
 differ from a Mac (12 and 571 there, the fast path without the Linux-only
 `getWindowState` call), so take J1 baseline values from the runner only.
-`renderer.layoutShiftScoreSettled` has no baseline either: it has only been
-measured on a Mac so far (see [Settle window](#settle-window)), and a
-baseline needs the runner's number once the dashboard flicker it reports
-is fixed.
+`renderer.layoutShiftScoreSettled` has no baseline either: the runner reads
+it as `stable: false` because the dashboard flicker it reports is a race
+there (see [Settle window](#settle-window)). Add the runner's number once
+that flicker is fixed and the counter is deterministic.
 
 ## Charset parse benchmark