perf(engine): prioritize pre-review scheduling for critical-path specs (fixes #476)

[mickume]

Summary

• Exempt reviewer:pre-review nodes from cross-spec dependency propagation so they run immediately at start rather than waiting for upstream specs to complete
• Add two-tier priority scheduling: auto_pre nodes (group 0) run before coder nodes, sorted by spec fan-out descending so critical-path specs surface blockers first
• Add _compute_spec_fan_out() to GraphSync for fan-out-weighted ordering

Closes #476

Changes

File	Change
agent_fox/graph/builder.py	Skip cross-spec dep propagation for reviewer:pre-review nodes
agent_fox/engine/graph_sync.py	Two-tier scheduling with _is_auto_pre, _spec_round_robin, _compute_spec_fan_out
tests/unit/engine/test_spec_fair_scheduling.py	14 new tests for pre-review priority, fan-out, integration
tests/unit/graph/test_builder.py	Updated cross-spec propagation tests
tests/property/engine/test_spec_fair_scheduling_props.py	Updated property tests for tiered invariants

Tests

• 14 new unit tests: pre-review priority ordering, fan-out ordering, _is_auto_pre detection, spec fan-out computation, ready_tasks integration
• 3 updated builder tests: pre-review exemption, drift-review still propagated
• 3 updated property tests: fairness, spec order, and duration ordering checked per-tier

Verification

• All existing tests pass: ✅ (5050 pass, 2 pre-existing unrelated failures)
• New tests pass: ✅
• Linter / formatter: ✅
• No regressions: ✅

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