Add benchmark suite and fix core rendering hot-path inefficiencies

[berendkleinhaneveld]

Summary

• Adds a pytest-benchmark suite (bench/) covering mount cost, nested-component mount, attribute-update-at-depth, and keyed/unkeyed v-for reconciliation, mirroring the sibling observ repo's setup.
• Adds .github/workflows/benchmark.yml: on PRs, runs the suite against master and against the PR branch, failing the job if mean time regresses >5%.
• Fixes three hot-path inefficiencies in the core rendering code, each verified against the new benchmarks and the full test suite:
  • weak.py: the arity check no longer builds a full inspect.Signature per callback, just reads __code__.co_argcount. ~25-33% faster mounting of elements with dynamic binds.
  • ComponentFragment.mount(): replaces a hand-rolled BFS/deque with the already-existing first() DFS helper. Added a regression test for the wrapped-root case.
  • Fragment._component_parent(): now cached instead of walking the parent chain on every attribute update. Flattens update_at_depth benchmark scaling from ~5/7.2/15.5µs (depths 10/50/200) to a flat ~4.3µs. Includes an explicit cache-invalidation hook for the one place a mounted fragment is silently reparented (<component :is="..."> tag switches), with a regression test (test_dynamic_component_tag_switch_updates_correct_owner) that fails without the invalidation and passes with it.

Test plan

• uv run pytest tests -q — 266 passed, 14 skipped (up from 264/14 baseline; two new regression tests added)
• uv run ruff check . / uv run ruff format --check . — clean
• uv run pytest bench --benchmark-only — all 36 benchmark cases pass
• Verified the _component_parent cache-invalidation regression test fails when the invalidation hook is removed, and passes when restored
• Confirm the new benchmark.yml workflow runs successfully in CI on this PR

🤖 Generated with Claude Code

[berendkleinhaneveld]

Superseding this with a split series so each change is proven independently by CI:

1. Benchmark suite + per-PR benchmark workflow (merged first, so subsequent PRs get automatic master-vs-branch comparison)
2. Mount-path micro-optimizations (weak() arity check, ComponentFragment.mount root lookup, Component.emit)
3. _component_parent() caching + invalidation
4. anchor() single scan + loop-invariant anchor hoist in unkeyed v-for updates (new O(n²)→O(n) fix not in this PR)

The sound fixes from this branch are carried over essentially verbatim. Keeping this branch around as reference until the series lands.