ORCHESTRATOR.md: document parallel-execution craft + recovery patterns

Fold in the working practices that were implicit:
- §2a Waves: disjoint+no-compile-dep batching; widen waves by removing edges;
  parallel = N summon calls in ONE message (resolves the no-background tension)
- §3: pre-author the cross-unit seam; tell agents about concurrent siblings;
  make agents implement (not deliberate)
- §4: whole-project-check concurrency caveat + double-run for shared-global flakiness
- §5a Recovery: plan-only agent re-summon; sibling test fan-out; lane-stray
  handling; flaky green
- §9 Live integration probe (gated scripts/live-probe.ts; backend is user's process)
- §10 Living backend-handoff.md as the cross-repo courier channel
- generalize contract mirroring to all .dispatch/*.reference.md; re-read rules
  before each wave (a miss this session)

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