Record the launch-window versioning convention's end condition — post-GA the fixed group returns to strict semver #14043

Maintainer-approved follow-up from the director decision batch B (2026-09-01, verbatim 「同意」 on the #13829 grade item, which carried this card as its companion).

What to record

The repo's launch-window convention — breaking changes on the lockstep 17.x fixed group ship as minor with a mandatory **BREAKING** banner and ADR-0087 disposition, never as major — is deliberate and consistently applied (12+ changesets, 3-for-3 precedent on IObjectQLEngine). What it lacks is a written end condition: without one, a window-period convention erodes into a permanent exception.

One paragraph, wherever the convention's home is judged to be (AGENTS.md's versioning guidance, or the changeset-gate header that already polices banners):

• The convention applies during the launch window only.
• End condition: at GA (the group's first general-availability major), the fixed group returns to strict semver — a required member on a published interface, an accept-set narrowing, or any compile-breaking change to implementers then grades major.
• Until then, the banner + ADR-0087 disposition are the mandatory information carriers for breaking-ness.

Notes for triage

• Routing by landing spot: if it lands in AGENTS.md this is a governed-face edit (draft-only, human merge); if in a gate header, domain:devx. Suggested: wherever check-changeset-* documents the convention today, so declared = enforced stays one place.
• S-mechanical once the landing spot is chosen; the substance above is ruled, not to be re-litigated.

Filed by the director seat, session session_015adLit3ZYASJiXwxKG78Wi. Grading/routing is triage's.