feat(fuzz): add structural reduction

[gaborbernat]

Character-only minimization can leave unrelated HTML elements or CSS declarations in saved findings. This adds subtree and attribute deletion for HTML, declaration deletion within parsed CSS rules, and JavaScript AST reduction for #1019.

The JavaScript reducer forks the pinned UglifyJS reducer with UglifyJS 3.19.3 and checks candidates through the original finding predicate. CSS reduction uses tinycss2 1.5.1 block parsing. Structural and character reduction share the 600-comparison limit across JSON fields. MSan and operation-VM TSan remain requirements for completing #1019.

[codspeed]

Merging this PR will degrade performance by 6.2%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 5 regressed benchmarks
✅ 576 untouched benchmarks
⏩ 32 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_feature[prune-shared-single]	88.7 µs	96.1 µs	-7.7%
❌	test_feature[text-content]	2.6 ms	2.8 ms	-6.21%
❌	test_feature[select-relative-sibling]	42.6 µs	45.3 µs	-6.1%
❌	test_feature[serialize-inner]	5.1 ms	5.4 ms	-5.64%
❌	test_feature[serialize-inner-indent]	5.5 ms	5.9 ms	-5.32%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing gaborbernat:feat/fuzz-reduce-1019 (e7a31c3) with main (59b515e)}

Footnotes

1. 32 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩