docs: add maintainer ladder/emeritus to governance, add RELEASE.md

[lavianalon]

What

Two governance/release documentation additions:

• GOVERNANCE.md — adds a "Becoming a maintainer" section (contributor -> reviewer -> maintainer ladder, nomination by public PR, confirmed by lazy consensus, open to any organization) and a "Stepping down and emeritus" section (how maintainers step down or are moved to emeritus).
• RELEASE.md (new) — documents versioning policy (SemVer, pre-1.0 caveats), release cadence, who can cut a release, and the requirement to document breaking changes with migration guidance. The mechanics already live in CONTRIBUTING.md; this covers the policy around them.

Why

Continues the OSS health hardening (after #111 and #125). A documented maintainer ladder and emeritus path are credibility steps toward vendor-neutral, foundation-track governance; a RELEASE.md documents the release contract for downstream consumers. No code changes.

Note for maintainers

OWNERS lists a fourth approver (Ronkahn21) who is not in MAINTAINERS.md. This PR does not resolve that drift, since whether that person is a maintainer is a call for the team. Worth reconciling in a follow-up.