feat(map-efficient): parallel-wave worktree merge coordinator (Part of #284 Phase 2)

[azalio]

Wires the existing wave/DAG scheduler to per-subtask worktree isolation (Phase 1, #296) so a parallel wave of independent subtasks each runs in its own worktree and is accepted atomically via a new runner-owned coordinator merge_wave_worktrees. This delivers the headline-open acceptance criterion of #284: Wave-based parallelism for independent subtasks.

The core problem

Every worktree of a wave is cut off the same base (HEAD at wave start), so they cannot be merged one at a time — the first merge_subtask_worktree advances HEAD and the next trips its strict BASE_DIVERGED guard.

Design (llm-council-reviewed, conv c29d6fa9)

• Dedicated coordinator, not a flag — zero blast radius on the shipped single-subtask path. The two share the extracted _wt_freeze_and_verify primitive (commit + per-worktree guards + pre-merge verify) but stay separate compositions. wave_base_sha is derived from the sidecar, never a caller parameter.
• Wave-scoped divergence guard — refuses EXTERNAL HEAD movement (EXTERNAL_HEAD_MOVED) but allows the sibling divergence each in-wave squash-merge creates.
• Merge by frozen SHA, sorted id order — one runner commit per subtask (one-commit-per-subtask contract holds).
• All-or-nothing — any textual conflict, commit failure, or post-wave-gate failure rolls the whole branch back to base via a hard reset + clean -fd (squash leaves no MERGE_HEAD, so merge --abort is never used; MAP runtime state is excluded from the clean) and leaves every worktree intact for retry.
• One post-wave full gate inside the atomic transaction — catches a semantic break two subtasks create together (A renames a symbol B references) that no textual merge can see.
• Safety extras — advisory flock serializes coordinators (MERGE_IN_PROGRESS); attached/clean-target preconditions; conflicted paths attributed back to the subtasks that touched them; actual-overlap telemetry (declared-disjoint affected_files is only a scheduler hint).

What changed

• Runner: merge_wave_worktrees + helpers (_wt_rollback, _wt_unmerged_paths, _wt_changed_files, _wt_overlap_pairs, _wt_attribute_conflict, advisory-lock helpers) + CLI dispatch; refactored merge_subtask_worktree onto the shared _wt_freeze_and_verify primitive (single-subtask behavior byte-identical — all 30 existing tests pass).
• Skill wiring: map-efficient SKILL.md + efficient-reference.md (claude + codex variants).
• Docs: ARCHITECTURE Phase 2 section, USAGE parallel-waves, CHANGELOG.
• Tests: 12 wave-coordinator cases — happy path (sorted order, 2 disjoint subtasks), conflict full rollback + attribution + no MERGE_HEAD, external-head refusal, post-wave fail rollback / pass accept, preflight verify-fail abort-before-merge, no-change subtask, overlap telemetry, CLI happy + error-exit.
• Drive-by: widened build_minimality_rollout_report -> dict[str, Any], clearing 19 pre-existing pyright errors in tests/test_minimality_report.py (the dict[str,object] __getitem__ barrier).

Testing

make check green locally: ruff + mypy + pyright 0 (src AND tests), 2922 tests passed, render check (4 generated trees byte-identical). CI on this repo is currently down (billing), so the local gate is the authority.

Deferred (issue stays open)

• Phase 3 — context-budget hooks (statusline, threshold warnings, heartbeat).

Part of #284