feat: v0.3 query ownership and complexity fixes

[OrlovEvgeny]

Introduce a Query scope that owns everything a query produces, and remove the quadratic paths and recursion limits the new benchmark harness exposed.

Ownership:

• Add Query (src/query.zig) with its own scratch arena. Query results, chained Selection states, parsed selectors and the strings returned by html(), text() and outerHtml() now live and die with the query instead of the document, so repeated queries against a long-lived document no longer grow it. reset() reclaims a round while keeping capacity; DOM mutations stay owned by the document.

Complexity:

• wouldCreateCycle ran under an assert evaluated in every build mode, making tree building O(nodes x depth).
• autoClose scanned the whole open-element stack per start tag.
• :has() re-scanned the entire document per anchor.
• :nth-child() recounted a sibling list per candidate.
• Render, clone, validate, text extraction and matching now traverse iteratively, so deeply nested documents no longer overflow the stack.

Benchmarks:

• Add zig build bench (always ReleaseFast) with deterministic corpora, per-operation memory accounting and an empirical complexity check (--scaling), plus v0.2 and current baselines.
• CI gates the two things that are not noisy on shared runners: no benchmark may retain memory in the document arena, and no workload may scale superlinearly. zig fmt --check now covers bench/.