fix(workflows): isolate backlog grooming safe output per candidate

@rezatnoMsirhC is already working on this.

Since Sep 8, 2026.

The backlog-grooming worker currently serializes every candidate row in a shard into one scalar `report-data` JSON string. A single syntax error in that nested JSON prevents parsing before candidate identity can be bound, so the worker cannot safely retain the malformed candidate as a `contract_error` while continuing with valid candidates.

This occurred in [Backlog Grooming Sweep run 33838636334](https://github.com/microsoft/hve-core/actions/runs/33838636334): one malformed evidence value made the complete shard envelope unparsable. The validator correctly failed closed rather than guessing how to repair model-authored JSON, but the entire sweep wave stopped. Repeated custom safe-output calls are collected as independent items, so emitting one independently parseable result per planned candidate should reduce the failure boundary without weakening validation.

## Acceptance Criteria

- [ ] Each planned candidate is emitted through an independently parseable safe-output item, or an equivalent transport that prevents one candidate's JSON syntax error from invalidating unrelated candidates.
- [ ] Candidate identity is bound to exactly one planned shard issue before advisory fields are validated.
- [ ] A malformed candidate becomes a fixed non-advisory `{ issue, code }` diagnostic only when its identity is safely attributable; malformed or ambiguous identity still fails closed.
- [ ] Accepted rows, deferred rows, and contract-error issue IDs remain disjoint and exactly cover the planned candidate set.
- [ ] The implementation does not introduce a permissive JSON repair parser or infer missing model-authored punctuation, quoting, or boundaries.
- [ ] Aggregate, checkpoint, recovery, reducer, and publisher validation preserve strict provenance and continue blocking publication when contract errors are nonzero.
- [ ] Focused tests prove that one syntactically malformed candidate does not discard valid sibling candidates and that unbound malformed output cannot enter trusted state.
- [ ] The generated lock workflow is regenerated from its Markdown source and remains synchronized.

## Related

- #2827
- Original automated backlog grooming feature: #2558
- Failure evidence: https://github.com/microsoft/hve-core/actions/runs/33838636334