fix: prepare 0.1.0 stable release

[tensornull]

What changed

• Add three-state alpha/stable-promotion/stable-candidate release validation.
• Accept strict Keep-a-Changelog and Release Please v5 version headings.
• Execute the canonical README ESM and CommonJS examples against an exact tarball with fail-closed mocked networking.
• Configure Release Please to create only the reviewed stable PR and document the manual immutable release boundary.
• Run the latest-compatible OpenAI lane for manually dispatched release-candidate CI.

Why

The alpha line is complete, but stable publication needs version-aware public-content gates, exact-artifact example evidence, and a safe boundary around the open Release Please single-package tagging defect.

Validation

• npm run verify
• npm test (12 files, 204 tests)
• npm run test:examples
• npm run test:package
• npm run actionlint
• npm run check:public-preview
• three independent adversarial reviews