groomer: align apply-time head precondition with search-rechecked freshness #1116

Follow-up from #1091 (PR #1114).

Problem

After #1091, the freshness pass can advance groomingVerifiedSha for a global-scope issue when its saved empty searches still return nothing, treating the result as fresh across a default-branch move. But checkHead in src/lib/groomer/mutation-validator.ts unconditionally fails the head precondition for any head move on a global-scope issue. A plan re-applied after such a pass fails preconditions with outcome: stale even though the freshness pass just accepted the same evidence.

The two checks should agree on what counts as verified current state for global evidence with saved queries.

Scope

• Reuse the groomer freshness: re-check saved empty code searches instead of staling global results on every commit #1091 recheck logic (grace window, defer limit, budget, conservative fallbacks) in the apply-time head precondition, or explicitly justify a stricter precondition.
• Keep the precondition conservative when the recheck cannot run.
• Tests cover: fresh-pass-advanced SHA then re-apply succeeds; head moved within the grace window then re-apply is stale.

Expected files

src/lib/groomer/mutation-validator.ts

Out of scope: changing staleness reasons or the freshness pass itself.