ci: use shared Node version

[albert20260301]

Summary

• add .node-version as the shared Node version source for local development and CI
• pin the repo to Node 24.17.0 in the root package.json engines field
• make both test workflow jobs and the release workflow read Node from .node-version
• include .node-version in the CI dependency cache key so cached native modules cannot cross Node ABIs
• document the Node prerequisite in the root README

Impact

This keeps dev machines and GitHub Actions on one exact Node version. Bun remains the package manager/runtime for install, build, lint, and tests; Node is pinned for tooling that runs through Node directly, including npm publish.

Validation

• git diff --check
• bun run typecheck
• bun run build
• bun run lint
• bun run test
• GitHub Actions: title validation, test, and integration passed

[albert20260301]

@francoischalifour could you review this when you have a slot? This pins the repo to one Node line via .node-version and makes CI read from it.