chore: add native Vitest benchmarks and PR comparisons

[devlsh]

Summary

bench/ + bench.config.ts
  → native quick/full workloads
benchmark workflow
  → build base → capture → build head → compare

Add readable, deterministic benchmark fixtures and native reference/candidate files. Use a separate PR workflow with native Vitest console and GitHub Actions reporters. Comparisons remain informational, with no uploads, custom runner, or performance threshold.

Update contributor, development, and release documentation. Update existing CI workflows and remove the shared setup action. No production source files change. No related issue was supplied.

Alternatives rejected: custom benchmark orchestration and stored CI artifacts.

Evidence

• Before: The main baseline has no benchmark harness.
• After: Earlier session validation passed pnpm check, pnpm build, 20 library tests, 12 quick workloads, and 27 full workloads. A local paired full run produced 27 baseline and 27 candidate files. Native reporter validation produced comparison tables and a GitHub test summary for 12 quick cases, without changing baseline hashes. Actionlint passed for all four workflows.

These results precede the final user edits. The pushed commit was not re-reviewed or re-tested. Benchmark callbacks measure performance only; existing library tests own correctness. No new benchmark correctness tests or runtime assertions were added. Hosted CI results remain pending.

Merge Danger

Door: two-way

Blast Radius: CI

Review the paired built-output transfer, workflow setup changes, and shared-action removal. This PR does not dispatch a release or publish a package.

AI assistance contributed implementation and earlier independent reviews. The user made final edits and committed and pushed them. The assistant did not review those final edits, as requested.