feat: Phase 3 — event-driven market-making simulator + 53 tests

New sim/ module — 7 files + init, replays stored L2/trade data
through a realistic market-making simulation:

sim/engine.py (SimulationEngine):
  Event-driven core — processes L2 updates, trades, mark prices
  sequentially. Orchestrates queue model, maker quotes, fill sim,
  constraints, scenarios. Supports periodic re-quoting and
  stale order cancellation.

sim/queue.py (QueueModel):
  Price-time FIFO queue per price level. Tracks where maker orders
  sit in queue. Simulates order eating by aggressor trades.
  fill_probability() — Poisson thinning model for fill odds.

sim/maker.py:
  AvellanedaStoikovMaker — stochastic control quoting with
    aeta, k, tau parameters. Reservation price based on inventory.
    quote() and quote_with_skew() with configurable inventory tilt.
  GridMaker — evenly-spaced grid quoting at N levels.

sim/fills.py:
  FillSimulator — partial fills, adverse selection probability,
    cancel latency (gaussian RTT). FillEvent/CancelEvent tracking.
  adverse_selection_intensity() — measures post-fill price moves.

sim/constraints.py:
  InventoryConstraint — long/short/net/gross position limits.
  FundingConstraint — hourly funding cost estimation.
  FeeSchedule — maker/taker fee calculation.
  LiquidationRisk — liquidation price and safety distance.
  CircuitBreaker — PnL, trade count, toxic rate, slippage trips.
  ConstraintManager — unified pre-trade constraint check.

sim/scenario.py:
  ScenarioEngine — randomized exchange downtimes, latency spikes,
    volatility bursts. State query per sim_time for spread/trade-rate.

sim/reporter.py:
  PnLReporter — component-level PnL breakdown:
    spread_capture, inventory_pnl, fees, funding, adverse_selection.
  SimulationStats — trade counts, fill rates, drawdown, sharpe.
  Equity curve tracking and max drawdown computation.

53 new tests across 4 files (all pass):
  test_sim_queue.py (12) — order placement, FIFO, fills, cancels
  test_sim_maker.py (9) — A-S quotes, inventory skew, grid maker
  test_sim_constraints.py (14) — limits, funding, fees, liquidation, breakers
  test_sim_reporter.py (12) — PnL components, equity curve, stats
  test_sim_engine.py (6) — full engine integration

Total test suite: 134 tests, all passing.

tests/test_sim_constraints.py

@@ -0,0 +1,96 @@
+"""
+Tests for sim/constraints.py — inventory, funding, fees, liquidation, circuit breakers.
+"""
+from sim.constraints import (
+    InventoryConstraint,
+    FundingConstraint,
+    FeeSchedule,
+    LiquidationRisk,
+    CircuitBreaker,
+    ConstraintManager,
+)
+
+
+class TestInventoryConstraint:
+    def test_within_limits(self):
+        ic = InventoryConstraint(max_long=0.01, max_short=0.01)
+        result = ic.check(0.005, 0.003)
+        assert result["long_ok"]
+        assert result["short_ok"]
+        assert result["net_ok"]
+
+    def test_long_exceeded(self):
+        ic = InventoryConstraint(max_long=0.005)
+        result = ic.check(0.006, 0.001)
+        assert not result["long_ok"]
+
+
+class TestFundingConstraint:
+    def test_cost_positive_funding(self):
+        fc = FundingConstraint(funding_rate_hourly=0.0001)
+        cost = fc.cost_per_hour(position=1.0, mark_price=100000.0)
+        assert cost == 10.0  # 1 * 100000 * 0.0001 = 10
+
+    def test_annualized(self):
+        fc = FundingConstraint(funding_rate_hourly=0.0001)
+        ann = fc.annualized_cost_pct(0.0001)
+        assert ann == 0.0001 * 365 * 24 * 100
+
+
+class TestFeeSchedule:
+    def test_maker_fee(self):
+        fs = FeeSchedule(maker_fee_pct=0.0002)
+        assert fs.maker_fee(100000.0) == 20.0
+
+    def test_taker_fee(self):
+        fs = FeeSchedule(taker_fee_pct=0.0005)
+        assert fs.taker_fee(100000.0) == 50.0
+
+
+class TestLiquidationRisk:
+    def test_liquidation_price_long(self):
+        lr = LiquidationRisk(maintenance_margin_pct=0.03, initial_margin_pct=0.05)
+        liq = lr.liquidation_price(entry_price=100000.0, size=1.0, position_side="long", wallet_balance=10000.0)
+        assert 0 < liq < 100000.0  # below entry
+
+    def test_liquidation_price_short(self):
+        lr = LiquidationRisk(maintenance_margin_pct=0.03, initial_margin_pct=0.05)
+        liq = lr.liquidation_price(entry_price=100000.0, size=1.0, position_side="short", wallet_balance=10000.0)
+        assert liq > 100000.0  # above entry
+
+    def test_safe_distance(self):
+        lr = LiquidationRisk()
+        assert lr.is_safe(5.0, threshold_pct=2.0)
+        assert not lr.is_safe(1.0, threshold_pct=2.0)
+
+
+class TestCircuitBreaker:
+    def test_no_trip(self):
+        cb = CircuitBreaker()
+        result = cb.evaluate({"pnl_pct": -1.0, "daily_trades": 10, "toxic_rate": 0.1, "api_errors": 0})
+        assert not result["tripped"]
+
+    def test_drawdown_trip(self):
+        cb = CircuitBreaker(max_drawdown_pct=-5.0)
+        result = cb.evaluate({"pnl_pct": -6.0, "daily_trades": 10, "toxic_rate": 0.1, "api_errors": 0})
+        assert result["tripped"]
+
+    def test_trade_count_trip(self):
+        cb = CircuitBreaker(max_daily_trades=100)
+        result = cb.evaluate({"pnl_pct": -1.0, "daily_trades": 150, "toxic_rate": 0.1, "api_errors": 0})
+        assert result["tripped"]
+
+
+class TestConstraintManager:
+    def test_can_place_bid(self):
+        cm = ConstraintManager()
+        result = cm.can_place(side="bid", size=0.001, current_position=0.0, mark_price=100000.0)
+        assert result["allowed"]
+        assert round(result["fee_estimate"], 4) == 0.02  # 0.001 * 100000 * 0.0002 = 0.02
+
+    def test_cannot_exceed_inventory(self):
+        cm = ConstraintManager(
+            inventory=InventoryConstraint(max_long=0.002, max_short=0.002)
+        )
+        result = cm.can_place(side="bid", size=0.001, current_position=0.0015, mark_price=100000.0)
+        assert not result["allowed"]