verified: failure loop -- deterministic level, runnable issue, fix-PR linkage, same-pipeline close (#6655 Phase F) #6689

Part of #6655, Phase F (automatic failure loop). Stacked on #6671, which adds specs/verified/reuse.t27.

Goal

Make the failure loop deterministic:

• the base priority level a failure issue is filed at;
• what makes a failure issue runnable by the Queen;
• what a fix PR must reference;
• when the loop may close.

Where we are

• specs/verified/reuse.t27 (feat(verified): reuse/rebuild rule and failure fingerprint (Phase A of #6655) #6671) has the fingerprint, on_failure (open or update) and on_fix_run (close only on a passing, sealed, non-recurring run).
• specs/queen/priority.t27 has the levels, the critical cap and aging.
• Nothing decides a failure's level, requires a fix PR to link back, or requires the same pipeline to be re-run.

Boundary

• New file specs/verified/failure_loop.t27 plus its seal. It imports verified::reuse and mirrors the priority.t27 level values.
• No occurrence thresholds, no restated cap or aging, no non-t27 code.

User Scenarios & Testing

• A failure on the default branch: CRITICAL. A fingerprint that recurred after its issue was closed: HIGH. Anything else: NORMAL.
• A failure issue missing its Boundary, Success Criteria or fingerprint is not runnable.
• A fix PR that does not reference both the failure issue and the original change is not linked.
• Unlinked fix or a different pipeline: the issue stays open (KEEP). A recurring fingerprint: UPDATE. Linked + same pipeline + passed + sealed: CLOSE.

Requirements

• failure_level(on_default_branch, recurred_after_close) -> u8
• runnable(has_boundary, has_criteria, has_fingerprint) -> bool
• fix_linked(refs_failure_issue, refs_original_change) -> bool
• close_action(linked, same_pipeline, passed, seal_verified, recurred) -> u8, delegating to reuse.on_fix_run

Success Criteria

• t27c gen specs/verified/failure_loop.t27 > x.zig && zig test x.zig: 10/10.
• Every guard has a mutant that the tests kill.
• t27c seal --verify: all hashes MATCH.
• gen-verilog and gen-c succeed.