exec(unified): L2 foundation — contract §2 + whether-maker Router §6, pure, 26 tests

First code of the Unified Execution Layer (SPEC_UNIFIED_EXEC_LAYER_20260714.md). Pure
policy, stdlib+Decimal only, zero I/O, zero venue knowledge, zero importers elsewhere —
adopting it breaks nothing (freeze-safe; operator unparked the build 2026-07-14).

- contract.py: ExecutionRequest input surface (§2.1) + V-TYPES (Side, UrgencyClass,
  ProtectiveSpec, ExecutionAdvice), frozen, validated-at-construction, illegal states
  unrepresentable. 'an asset, a size, and a prayer'.
- router.py: decide(request) -> RoutingDecision — total pure map of the §6 urgency
  ladder. Encodes A1 adjudication verdict in the architecture: Router=whether-maker,
  SmartPlacer=where-in-book via the wants_placement/pre_submit seam. Complementary.
- _constants.py: policy magnitudes with provenance; PROVISIONAL ones flagged for
  L8/L10 calibration (never vibes, never a hardcoded cadence).
- test_exec_unified.py: 26 behaviour + mutation-litmus tests. Verified RED under
  CATASTROPHIC->MAKER and ACQUIRE cross_on_expiry->True mutations.

Not yet wired: PINK drive-loop port (§7), venue dialect (§11), telemetry (§12).

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

prod/exec_unified/_constants.py

@@ -0,0 +1,30 @@
+"""Policy constants — from data, never vibes (spec §6 closing note).
+
+Every constant cites its provenance. Where a number is not yet backed by an L8/L10
+characterization row it is marked PROVISIONAL and must be calibrated before the smart
+path arms with real capital — a provisional constant is a known-unknown, not a vibe.
+
+Cadence is NEVER hardcoded here (no literal 1 s): the drive loop takes its clock
+injected (spec §7, C1). These are policy magnitudes, not periods.
+"""
+from __future__ import annotations
+
+from decimal import Decimal
+
+# PROTECT: "maker at touch, one reprice, ≤2 s total, then cross" (spec §6).
+PROTECT_REPRICE_LIMIT: int = 1
+PROTECT_MAX_MS: int = 2_000
+
+# HARVEST: "maker at target ± offset, chase ≤ N; give up when regression eats X%
+# of unrealized → cross" (spec §6). N and X are PROVISIONAL — no characterization
+# row fixes them yet (§17 open question: chase bounds from bookDepth replay).
+HARVEST_MAX_CHASES: int = 3                              # PROVISIONAL — calibrate vs L8 fill_sim
+HARVEST_GIVEUP_REGRESSION_FRAC: Decimal = Decimal("0.25")  # PROVISIONAL — 25% of unrealized
+
+# ACQUIRE: "patient maker inside spread; abandon, never chase" (spec §6, §4-1:
+# a missed entry is free, a chased entry is not). No chase budget by construction.
+ACQUIRE_MAX_CHASES: int = 0
+
+# Dead-man's stop default multiple of software SL distance (spec §10, §17 open
+# question: 2× vs ATR-scaled). PROVISIONAL default.
+PROTECTIVE_STOP_MULT_DEFAULT: Decimal = Decimal("2.0")