feat(valgrind): add --experimental-rep-bulk-cost flag - #620

Merging this PR will not alter performance

✅ 53 untouched benchmarks
⏩ 6 skipped benchmarks¹

_{Comparing feat-experimental-rep-bulk-cost (f4a85b0) with main (34b48f1)}

Confidence Score: 4/5

[Medium impact] The change reaches benchmark runs that opt into the new Callgrind cost model.

The code changes appear safe to merge once the already acknowledged Valgrind release prerequisite is satisfied; the test suggestion is non-blocking.

Findings

1.  Cost model conditions lack tests ▶

### Issue 1
src/executor/valgrind/measure.rs:50-52
The new `get_valgrind_args` branch has no tests; the existing tests only cover the shell wrapper. Add checks that `--rep-bulk-cost=yes` appears only for Callgrind when both settings are true, and stays absent by default, with cycle estimation disabled, and with Tracegrind. This would catch future changes that silently alter benchmark costs.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Summary

Adds opt-in --experimental-rep-bulk-cost and CODSPEED_EXPERIMENTAL_REP_BULK_COST support.

• Benchmark runs can opt into Callgrind's calibrated REP-copy cost model.

Known prerequisite acknowledged by not-matthias: publish a compatible Valgrind release and bump VALGRIND_CODSPEED_ITERATION before merging, because older builds reject the option.

_{Reviews (1) · Last reviewed commit: "feat(valgrind): add --experimental-rep-b..." · Reviewed by Greptile}