B1: deliverable_keys answered a multiset, so a `+` title whose segments
normalize to one key made collision_flags find the issue adjacent to
itself and chain it to its own number — the comment asked an issue to
declare `Blocked by` itself, and two such carriers corrupted the chain
between them. The keys are deduped where the set property belongs.
B2: window_in_scope excluded only blocked/epic/post-merge, admitting
`needs-triage` and label-less issues, so the sweep could add
`needs-triage` to an issue and then tell it about a mint-time membership
call in the same pass. Both flags now call one unblocked_claimable
predicate — #293 D2 corrected gives one gloss on `unblocked` and D3b
says D3 uses it.
The window log line says "unblocked", not "ready": D3b corrected exactly
that wording, and the flag fires on `claimed` too.
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