Benchmark installed-wheel indexing across pool sizes

[zaniebot]

Installed-wheel indexing has a size-based serial path and a configured installer pool, so comparisons need controlled sidecar presence, sizes around the cutoff, and a separate process for each pool width. Build on #1444 and reuse the missing, present, and mixed sidecar fixture from #1493. Add adjacent cutoff sizes, an explicit per-process pool setting, and the observed width for explicitly configured pools. Construct only selected fixtures outside the measured operation, and provide a shared fixture exporter for fresh-process uv pip list comparisons.

[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks
🆕 22 new benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	WallTime	missing[1023]	N/A	37.2 ms	N/A
🆕	WallTime	missing[1024]	N/A	15.1 ms	N/A
🆕	WallTime	missing[1025]	N/A	15.3 ms	N/A
🆕	WallTime	missing[2000]	N/A	30.3 ms	N/A
🆕	WallTime	missing[50]	N/A	1.7 ms	N/A
🆕	WallTime	missing[500]	N/A	17.4 ms	N/A
🆕	WallTime	mixed[1023]	N/A	44.4 ms	N/A
🆕	WallTime	mixed[1024]	N/A	16.2 ms	N/A
🆕	WallTime	mixed[1025]	N/A	16.5 ms	N/A
🆕	WallTime	mixed[2000]	N/A	32.7 ms	N/A
🆕	WallTime	mixed[50]	N/A	2.1 ms	N/A
🆕	WallTime	mixed[500]	N/A	21.6 ms	N/A
🆕	WallTime	present[1023]	N/A	78.8 ms	N/A
🆕	WallTime	present[1024]	N/A	22.7 ms	N/A
🆕	WallTime	present[1025]	N/A	22.8 ms	N/A
🆕	WallTime	present[2000]	N/A	45.1 ms	N/A
🆕	WallTime	present[50]	N/A	3.7 ms	N/A
🆕	WallTime	present[500]	N/A	38.3 ms	N/A
🆕	WallTime	prune_retained_archive_files[10000]	N/A	96.7 ms	N/A
🆕	WallTime	prune_retained_archive_files[100000]	N/A	876.4 ms	N/A
...	...	...	...	...	...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

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_{Comparing zb/future-20260912-cache-index-benchmark-01 (03611af) with zb/future-20260912-cache-13 (e6e5a9d)}