docs: add GOVERNANCE — roles, change classes, promotion, succession

[y1o1]

External review: the spec self-describes as "maintainer discretion; discussions are public in Issues" with no GOVERNANCE.md — the "who decides, and what happens if they disappear" questions were unanswered.

GOVERNANCE.md adds the missing differential (deliberately not duplicating VERSIONING.md or CONTRIBUTING.md):

• Roles: single maintainer (1o1 Co. Ltd.) named, with the namespace publish root ownership stated.
• Change classes: editorial / additive / semantic / namespace, each with its required procedure; namespace paths are frozen-once-shipped.
• draft → stable promotion: requires the rule's vectors to pass in two independent implementations — the criterion doubles as a standing invitation for a second implementation.
• Succession: six-month unresponsiveness procedure, explicit fork-friendliness, and the note that offline verification survives the namespace (sha256-pinned embedded contexts).
• IPR: Apache-2.0 inbound=outbound with its patent grant; DCO explicitly deferred, not required today.

README status note now points at GOVERNANCE.md and names the known implementation pair (provin OSS v0.3.0 ⇄ spec v0.1 draft, vendored + CI-gated vectors).

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