feat(fuzz): bound XML grammar expansion

[gaborbernat]

XML lexical productions need an expansion bound without charging fictitious DOM nodes. Add separate node and step budgets to the shared generator, with reserved sibling costs and forced production paths for #1018.

Generate XML document families from XML 1.0 Fifth Edition and the pinned libxml2 dictionary, with literal tree and parent expectations. The corpus command writes exact well-formed XML bytes and a separate bounded raw-input profile. DTD resolution and XML1.1 remain outside this public parser profile.

[codspeed]

Merging this PR will degrade performance by 6.14%

⚠️ 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

❌ 4 regressed benchmarks
✅ 577 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	96.5 µs	-8.09%
❌	test_feature[serialize-inner]	5.1 ms	5.4 ms	-5.63%
❌	test_feature[form-data-fieldsets]	31.1 µs	32.9 µs	-5.52%
❌	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.

---

_{Comparing gaborbernat:feat/xml-grammar-1018 (d61ee7e) 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. ↩