Selector performance: :has() from the document root dominates matching cost — handover plan (existence index, selector cache, memoization)

[jvmvik]

--- deleting I'll create a clean benchmark from scratch

[jvmvik]

Branches referenced above are now pushed: lexbor-v3 (binding on lexbor 3.0.0; lexbor edits are captured in patches.v2-pin/ALL-PATCHES-ON-v3.0.0.diff, reproduction steps in LEXBOR_V3_PORT_NOTES.md) and aarch64-linux-ci (arm64 source-build + ASAN CI job, native-gem smoke test before publish, use_sigaltstack=0 fix for test:asan on arm64). Suggest merging aarch64-linux-ci first so the work here is tested on both architectures.

[jvmvik]

PRs opened for the two branches: #41 (aarch64 CI, ready for review — suggest merging first) and #42 (lexbor v3.0.0 port, draft — 305/307, serializer parity and template patches outstanding).