perf: optimize compiler analysis phase

[MathiasWP]

Summary

Two optimizations to the compiler's analysis phase:

• Cache ignore_stack snapshots instead of structuredClone on every node. The universal _ visitor in the analysis walk runs on every AST node and calls structuredClone(ignore_stack) each time. In practice, svelte-ignore comments are rare (0–5 per component), so 99%+ of nodes deep-clone an unchanged stack. This adds a copy-on-write cache that only re-creates the snapshot when push_ignore/pop_ignore actually change the stack.

• Walk the CSS stylesheet once instead of once per element. prune() was called in a loop for each element, each time doing a full walk() of the stylesheet AST. This restructures the loop so the stylesheet is walked once, and the element iteration happens inside the ComplexSelector visitor.

Benchmarks

Compiled each component 500 times (after 50 warmup iterations), measuring average time per compile() call:

Component	Before	After	Speedup
has (80+ CSS selectors, 12 elements)	3.405 ms	2.680 ms	21% faster
siblings-combinator-each-nested (65 CSS rules, 15 elements)	2.034 ms	1.575 ms	23% faster
synthetic (100 CSS rules, 50 elements)	10.099 ms	4.564 ms	55% faster

The CSS pruning optimization scales with elements × CSS rules — the more elements a component has, the bigger the win since we go from N stylesheet walks down to 1. The structuredClone fix helps every component regardless of CSS, eliminating ~500–2000 deep clones per compile (one per AST node) and replacing them with 0–5 (one per svelte-ignore comment).

For typical real-world components with 10–20 elements and some CSS, expect roughly 20–30% faster compilation in the analysis phase.

Test plan

• Full test suite passes (7329 tests, 0 failures)
• CSS pruning tests pass (selector matching, scoping, unused rule detection)
• svelte-ignore behavior unchanged (snapshot is consumed read-only via .has()/.some())

🤖 Generated with Claude Code

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🦋 Changeset detected

Latest commit: 41dbcc2

The changes in this PR will be included in the next version bump.

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[Rich-Harris]

fantastic!