abi: 30-day green burn-in monitor on release-1.x for v1.0 ABI golden file

[justinjoy]

Context

Decomposed from #690. The original issue's acceptance criterion was:

CI gate green for ≥30 consecutive days on release-1.x.

The release-1.x branch does not exist yet — it is cut at v1.0-rc1 per #685. So this 30-day burn-in clock cannot start until the rc1 cut. This issue tracks the monitor that confirms the ABI gates stay green throughout the 1.0 freeze window.

Scope

1. Once release-1.x is cut (Epic: v1.0.0-rc1 Freeze #685), enable a recurring GitHub Actions workflow (.github/workflows/abi-burn-in.yml) that runs the full meson test --suite abi matrix daily against the tip of release-1.x and records pass/fail in an artifact or a status-check.
2. Track green/red days; an automated comment on this issue updates the running tally weekly (or manually if automation is undesired).
3. Issue closes when 30 consecutive green days are recorded.

Acceptance criteria

• release-1.x exists.
• abi-burn-in.yml workflow is committed and active.
• 30 consecutive daily green runs of meson test --suite abi recorded.

Out of scope

• Re-running the burn-in if a single red day occurs mid-window — restart-on-failure policy is at maintainer discretion.
• Extending the window beyond 30 days — that's a v1.x consideration.

References

• Parent: was Blocker B3: ABI golden file infrastructure (#471) #690 (decomposition).
• Cross-link: Epic: v1.0.0-rc1 Freeze #685 (release-1.x cut, v1.0.0-rc1).
• Sibling Linux gates: libabigail .abi.json + arm64 enrollment (v0.41).

[justinjoy]

Codebase/milestone audit note for #685: this issue appears to be post-cut burn-in evidence, not the rc1 branch-cut/freeze decision itself. The explicit ABI freeze declaration is now tracked separately in #895.

Because this issue's own scope starts "Once release-1.x is cut" and closes only after 30 consecutive daily green runs, I am moving it to v1.0.0-rc2 Validation, where post-rc1 validation evidence fits better. #685 now references it as a post-cut/later validation tracker rather than the freeze-time ABI finalization item.