exec(unified): UnifiedExecutor — initial-submit pipeline + S2 pain-fence

executor.py composes the built parts into one execute(request, snapshot): S-grade fence →
router.decide → placer.pre_submit → submit → register working. The initial-submit half that
complements DriveLoop's expiry half (analogue of pink_direct.py:1645-1700).

Composition rulings encoded + tested:
- S2/S3 pain-fence (§15.2): refused WHOLE, loudly, below T14 — never silently sliced.
- placer declines (spread gate) → cross ONLY if urgency crosses on expiry; ACQUIRE ABANDONS
  (a missed entry is free, §4-1). Both mutation-verified RED.
- TTL from urgency discipline: PROTECT 2s / ROTATE deadline_ms / ACQUIRE quote-lifetime.

contract.py: SGrade enum (S0-S3) + s_grade/parent_request_id fields. _constants: MAKER_QUOTE_TTL_S.

FIX (real bug, not just test): drive_loop._is_resolved EXIT was trade_id-based (a PINK
artifact — PINK reused the position's trade_id for exits). The agnostic layer never gets
the position id, so exit-done is now SIZE-based. Kept ENTER on clientOrderId match.

Full exec_unified suite: 94 green, mutation-litmus verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

prod/exec_unified/__init__.py

@@ -13,19 +13,25 @@ from .contract import (
     ExecutionAdvice,
     ExecutionRequest,
     ProtectiveSpec,
+    SGrade,
     Side,
     UrgencyClass,
 )
+from .executor import ExecuteResult, ExecuteStatus, UnifiedExecutor
 from .router import ExecutionMethod, RoutingDecision, TtlDiscipline, decide
 
 __all__ = [
+    "ExecuteResult",
+    "ExecuteStatus",
     "ExecutionAdvice",
     "ExecutionMethod",
     "ExecutionRequest",
     "ProtectiveSpec",
     "RoutingDecision",
+    "SGrade",
     "Side",
     "TtlDiscipline",
+    "UnifiedExecutor",
     "UrgencyClass",
     "decide",
 ]