Why: Q1 WP1/WP2 - replace the linear step scan with per-round graph
compilation and layer advancement so Needs drives scheduling and
write-conflicting steps never run concurrently.
Evidence: go vet clean in all five modules; go test ok everywhere,
including the full compatibility net (engine, leader, recovery, r2
regression, intent, evaluator suites) and new graph tests.
Impact: new squad/graph.go (compile/layer/conflict); StepSpec gains
Writes; advance advances by superstep (parallel run of
conflict-free members, serial for write-conflicts, single-member
groups keep dynamic adjustment). Legacy semantics preserved:
adjacent-Parallel mapping, degraded-continue on non-critical
failure (failed counts as satisfied), empty status = pending,
cycle/unknown/dup needs fail-closed.
Refs: Documents proposal 2026-10-07-agent-stack-n1q1i1-plans.md Q1 WP1/WP2
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