feat(sdlc): worklist build EdgeGraph cutover (Phase 7 #7)

[PolyphonyRequiem]

Phase 7 #7 — worklist build EdgeGraph cutover

Replace the BFS-by-depth wave logic in polyphony worklist build with the
EdgeGraph.ToWaves() + ExecutionModeInjector composition, so worklist
generation goes through the same wave-derivation engine as edges check
and the apex driver. Hard cutover — no opt-in flag, no backcompat shim.

This unblocks apex-driver.yaml (the next worklist consumer) being able
to trust a single source of truth for wave shape across the SDLC.

Behavior changes (wire shape)

WorklistResult gains three required fields, present on every envelope
including errors:

• items_walked (int) — count of items in the subtree the verb walked.
• has_conflicts (bool) — true iff EdgeGraph surfaced any conflicts
  (cycles, unknown items, etc). Always present, even on error envelopes,
  so workflow consumers don't need to first distinguish error-vs-conflict.
• conflicts (array of EdgesCheckConflict) — same wire shape as the
  edges check verb (kind, description, contributing_edges).
  Empty array when no conflicts.

When has_conflicts: true, waves is [] (empty) — the consumer is
expected to surface the conflicts instead of acting on a partial wave plan.

waves[].wave_index was already the field name — no rename was needed
(the design doc's call to "rename depth → wave_index" was based on a
misread of the existing model). WorklistWave and WorklistItem shapes
are unchanged.

Behavior changes (semantics)

• Wave derivation now uses EdgeGraph.ToWaves() instead of BFS-by-depth.
  For the canonical Epic→Issue→Task definitional plan tree the wave
  topology coincides with BFS depth (children-unblock cross-item edges
  gate each child on parent's children_seeded), so existing depth-shape
  tests still hold logically. The new path additionally honors execution
  mode (sequential injects sibling chain edges, parallel does not)
  and plan_then_implement cross-item gating where applicable.
• Missing root: the old verb tolerated a missing root and emitted
  plan_status: unknown while still walking. The new verb errors with
  error_code: root_not_found because EdgeGraph composition needs the
  type from process config to derive the per-item requirement set. This
  is a deliberate semantic change consistent with the rest of the SDLC
  surface area (edges check behaves the same way).
• New error codes: root_not_found, type_unknown, derivation_failed,
  cache_error, graph_invalid. All emitted with exit 0 and a populated
  EmptyResult() envelope (items_walked: 0, waves: [],
  has_conflicts: false, conflicts: []) per the established
  Polyphony JSON contract.

What's NOT in this PR

• No --depth flag. The current worklist build signature does not
  expose --depth (the brief mentioned it both as "preserve" and "do not
  confuse with the JSON field" — best read of the surface area is that
  the flag never landed). Verb signature is preserved exactly:
  (rootId, manifestPath, json, ct). If a depth cap is wanted later it
  can be added cleanly on top of the new composition.
• No apex-driver.yaml changes. That's the next consumer to retrofit
  in a follow-up PR; this PR just lights up the worklist surface.
• No documentation backcompat notes. Per "hard cutover" guidance.

Verification

• dotnet build src/Polyphony/Polyphony.csproj — clean (0 warnings, 0 errors).
• dotnet test tests/Polyphony.Tests/Polyphony.Tests.csproj — 2707 / 2707 pass.
• New tests in WorklistCommandsBuildTests:
  • Build_EmptyTree_OneWaveOneItemNoConflicts
  • Build_WithExecutionMode_Parallel_DefaultBehavior
  • Build_WithPlanThenImplementItem_TwoPhaseGating
  • Build_EnvelopeShapeLockIn_AllExpectedSnakeCaseKeys
  • Build_EnvelopeShapeLockIn_HasConflictsAlwaysPresentEvenOnError
  • Build_WithCycle_EnvelopeShape_ContractLockIn (model contract; e2e
    cycle coverage stays at EdgeGraphTests per the edges check
    precedent — verb-built input maps can't cycle by construction)
  • RootMissingFromTwig_EmitsRootNotFoundError
  • UnknownType_EmitsTypeUnknownError
• New tests in JsonOutputContractTests:
  • WorklistBuild_SnakeCaseFieldNames_PresentInRawJson
  • WorklistBuild_DeserializationRoundTrip_FieldsMapped
  • WorklistBuild_NotFound_ReturnsErrorEnvelope_WithSuccessExitCode
• Pre-existing manifest discovery and human-render tests preserved.
• Pre-existing RootMissingFromTwig_EmitsUnknownStatusButStillWalks
  test replaced by the RootNotFoundError variant per the semantic
  change above.

Verified-by

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Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com