feat(engine): refuse work-unit-level fields that shadow the phases tree

[leeovery]

Summary

• Verified live: manifest set pay specification.foo bar succeeded and created a root specification: {foo: "bar"} object beside phases — an unvalidated shadow tree invisible to every phase-level read. The typo class ({wu} specification.x for {wu}.specification.{topic} x) landed garbage silently.
• set (both grammars), push, and apply set-ops now refuse work-unit-level fields whose first segment is a phase name, with a pointer to the canonical dot-path. Reads and delete stay free so a stray tree can be inspected and repaired.
• phases.*-headed fields stay permitted — they resolve into the real tree through full validation (and the contract suite pins that for fixture setup).

Test plan

• New suite in test-engine-manifest-fields.cjs: refusal in both set grammars + push + apply (nothing persisted), repair paths free, phase/topic-level and legit root fields untouched.
• npm test 1575 pass; test:cli, typecheck 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 fix(shared): strip triage-landing's unreachable superseded-research arm #519
15. feat(engine): refuse work-unit-level fields that shadow the phases tree #520 👈 current
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