docs: record alpha.3 registry release

[tensornull]

What changed

• mark Registry Alpha complete in the roadmap and README
• record the alpha.1 bootstrap recovery, unpublished immutable alpha.2 attempt, and successful alpha.3 OIDC release
• preserve exact CI, release, integrity, Sigstore, owner, dist-tag, and public-install evidence
• record latest -> 0.1.0-alpha.1 as residual cleanup before stable publication

Verification

• npm run verify
• 176 tests passed
• public-preview, standalone-content, package, fixture, compatibility, secret, self-containment, and actionlint gates passed
• independent public-registry ESM, CommonJS, and compatible OpenAI host installs passed before this evidence update