ci(bench): pin fork-JVM allocation ergonomics — B/op counted TLAB-retire waste, not just allocation

[kryptt]

Follow-up to #116 (merged in 6809e732): the TLAB-ergonomics pinning landed on that branch ~15 minutes after the merge, so it never reached main. This PR carries it — cherry-picked, applies clean on current main.

Why

gc.alloc.rate.norm counts TLAB-retire waste, whose size adapts to per-JVM heap/TLAB ergonomics — so the A/B profile's B/op "gate" metric was measuring environment state, not just allocation. Measured A/A on identical source (bench sources byte-identical on both sides):

configuration	worst A/A B/op delta on the flagged rows
-f 1 unpinned (the A/B profile)	+78–81% (PlatedBench.visitorUniverseJson n=4096: 295 KB ↔ 524 KB)
-f 3 unpinned	±11–41% — fork counts are the wrong axis: per-invocation environment state, not per-fork sampling noise
-f 1 pinned (-XX:-ResizeTLAB + fixed -Xms1g -Xmx1g)	±0.0% on n=4096, bit-identical n=512

CI-hardware validation

workflow_dispatch mode=aa on the pinned profile (run 35803316970), filtered to the flagged families — A/A noise floor from the run's own noise-report:

family	|ns/op Δ| p50 / p90 / max	|B/op Δ| p50 / p90 / max
OrderJsoniterBench	1.2% / 1.2% / 9.1%	0.000% / 0.000% / 0.002%
PlatedBench	6.7% / 6.7% / 7.7%	9.1% / 9.1% / 24.7%

vs the unpinned ±78–81% that produced the false alarms on #116.

What changed

• bench-pr.yml / bench-sweep.yml: -jvmArgsAppend -XX:-ResizeTLAB -jvmArgsAppend -Xms1g -jvmArgsAppend -Xmx1g on both JMH profiles; profile strings gain -rt1g so the R5 binding invalidates any stale calibration.
• bench_tools.py noise-report advice + README calibration step 2: pin the ergonomics before the bump--wi advice — fork counts don't fix this (measured).

Notes

• thresholds.json stays absent: the gate remains advisory until an N≥3 A/A calibration on CI hardware per the runbook; PlatedBench's 24.7% max on one pair says either sample more or raise -wi for that family first.
• Caveat: the pinned 1g heap must hold every swept bench's live set — if the nightly sweep OOMs, bump both files' -Xms/-Xmx together and recalibrate.

[github-actions]

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