test(perf): no tightening hint for an autoTighten:false baseline

Codex: the per-PR checker still printed "Lower the baseline … with this run
as evidence" for the J3 entry when a run read the low end of its band (731),
the change the flag exists to prevent. Such an entry now passes with the
reason instead, and is not counted as tightenable.

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

docs/architecture/performance-journeys.md

@@ -1228,7 +1228,9 @@ that file:
   `journeys.<journey>.counters` and a wall-clock entry (one with
   `toleranceRatio`) only from `journeys.<journey>.wallClock`, so a value in
   the wrong section also counts as missing;
-- a measurement below its baseline passes and prints a "tighten" hint;
+- a measurement below its baseline passes and prints a "tighten" hint,
+  except for an entry with `"autoTighten": false`, whose band one low run
+  does not move;
 - a measured counter without a baseline is noted, not failed;
 - checking nothing fails: an empty baselines file, or `--only` naming an
   entry that does not exist, cannot exit 0;