fix(shared): strip triage-landing's unreachable superseded-research arm

[leeovery]

Summary

• Chesterton check first: section F was added deliberately in r3: discovery and sessions — honest classification, visible agents #451's honest-classification pass, not legacy — but it models spec-style supersession (single absorber, superseded_by) that research supersession doesn't have.
• The only producer of superseded research is workflow-legacy-research-split's apply.cjs, which renames the item to {source}-superseded-{datetime} (never matching a map item, so triage can never target it) and supersedes via raw set status — superseded_by is never written. F was unreachable, and would interpolate an empty absorber name if it ever fired.
• Stripped the classification branch and section F; renamed G→F. A superseded item now falls to the neither-live branch → Fresh Target, whose existing ok: false handling surfaces the engine's topic start refusal verbatim — the code guard is the handler.

Test plan

• Conventions lint green.

🤖 Generated with Claude Code

Stack

1. feat(engine): discovery-map add-batch — the harvest persists in one call (D7) #505
2. feat(engine): manifest apply — batched set/delete across one work unit (D7) #506
3. refactor(prose): task-map family folds onto existing batch forms (D7, stage 3) #507
4. feat(engine): manifest set — two grammars, never mixed #508
5. refactor(prose): dump/read folds — subtree gets replace per-topic loops (D7, stage 5) #509
6. feat(engine): commit --plan — plan storage stages from recorded state (D7, stage 6) #510
7. fix: batching-stack review round — re-resolve rewired, guards symmetric, hardening pinned #512
8. fix: deep-dive round 2 — backbone rule flipped, pre-upgrade paths routed, flow bugs closed #513
9. fix(review-r3): engine hardening + crash-resume seams #514
10. feat(scoping): promotion routes through first-phase choice + bridge #515
11. fix(spec): pin the bugfix source name to the topic #516
12. fix(skills): gate-skip sentence accurate in gate-less phases #517
13. fix(review-entry): name the real upstream — specification and plan #518
14. fix(shared): strip triage-landing's unreachable superseded-research arm #519 👈 current
15. feat(engine): refuse work-unit-level fields that shadow the phases tree #520
16. fix(discovery): first-written briefs join the full brief lifecycle #521
17. test(sim): pipeline simulation — engine driven end-to-end as a black box #522
18. fix(round-4): review findings — backfill keying, ratchet evasion, pull guard #523