perf: add conditional rule CodSpeed benchmark

[hardfist]

Summary

Adds a CodSpeed benchmark fixture for long runs of adjacent conditional rules.

This introduces stylesheet/transform/conditional-run, which generates repeated adjacent @media, @supports, and @container blocks. The benchmark is intended to establish a baseline before applying the minification optimization in the follow-up PR.

Validation

• cargo check --benches

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks
🆕 2 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	stylesheet/parse[conditional-run]	N/A	10.5 ms	N/A
🆕	stylesheet/transform[conditional-run]	N/A	276.2 ms	N/A

---

_{Comparing perf/conditional-rule-codspeed-bench (504c3ed) with master (a55b05d)}

[kdy1]

I think this PR is good to go as it shows the performance difference, although Codspeed UI does not support this case.

ref: #1263

276.2 ms vs 13.9 ms

[hardfist]

codspeed support manually compare two branches and it's 20x speed up for this case https://app.codspeed.io/parcel-bundler/lightningcss/runs/compare/6a2688251fe9329263c497ff..6a23875a8d3fc83aa8269e47?uri=benches%2Ftransform.rs%3A%3Abenches%3A%3Abench_stylesheet_transform%3A%3Astylesheet%2Ftransform%255Bconditional-run%255D&runnerMode=Simulation