feat: Phase 2 — microstructure analytics + 81 tests

New microstructure/ module with pure-function analytics:

microstructure/book.py:
  microprice() — depth-weighted mid price
  mid_price() — simple bid/ask midpoint
  order_book_imbalance() — ranged [-1, 1] volume skew
  depth_imbalance() — imbalance at fixed price distance
  spread_stats() — spread, spread_bps, mid, bid, ask
  depth_resiliency() — bid/ask volume within impact radius
  queue_depletion_prob() — Poisson fill probability at level
  batch_book_stats() — aggregate stats across snapshots

microstructure/trades.py:
  classify_lee_ready() — Lee-Ready aggressor classification
  classify_bulk_lee_ready() — batch classification with mids/bids/asks
  compute_markouts() — forward mid-price change at configurable horizons
  markout_summary() — mean/std/t-stat per side per horizon
  trade_volume_profile() — size bucket distribution
  trade_arrival_rate() — rolling trades/sec with burst detection

microstructure/toxicity.py:
  compute_vpin() — volume-synchronized informed trading probability
  compute_vpin_time_series() — rolling VPIN with alarm threshold
  fill_toxicity() — adverse price movement post-trade
  adverse_selection_ratio() — per-side adverse selection
  liquidation_clustering() — cluster detection in liquidation events

microstructure/funding.py:
  funding_regime() — classify regime (neutral/positive/negative/high)
  funding_predictability() — AR(1) autocorrelation analysis
  funding_carry_pnl() — cumulative carry PnL estimation
  basis_spread() — perp premium over spot (bps)
  basis_convergence_speed() — mean-reversion half-life via AR(1)

microstructure/signals.py:
  composite_signal() — weighted OBI + trade + VPIN + funding signal
  SignalPipeline — stateful pipeline accumulating book/trade updates
  detect_hft_regime() — regime classifier for HFT strategy selection

Bug fixes in Phase 1:
  - data/latency.py: proper linear-interpolation percentiles
  - data/normalizer.py: UTC timezone for naive datetimes
  - data/normalizer.py: detect_sequence_gap returns gap-1 (missing count)
  - microstructure/toxicity.py: consistent vpin_value key in compute_vpin

81 tests across 4 test files (store, normalizer, latency, microstructure)

microstructure/__init__.py

@@ -0,0 +1,59 @@
+"""
+Microstructure analytics — book, trade, toxicity, funding, and signal composition.
+
+Pure functions that take market data arrays and return computed metrics.
+"""
+
+from microstructure.book import (
+    microprice,
+    mid_price,
+    order_book_imbalance,
+    depth_imbalance,
+    spread_stats,
+    depth_resiliency,
+    queue_depletion_prob,
+    batch_book_stats,
+)
+
+from microstructure.trades import (
+    classify_lee_ready,
+    classify_bulk_lee_ready,
+    compute_markouts,
+    markout_summary,
+    trade_volume_profile,
+    trade_arrival_rate,
+)
+
+from microstructure.toxicity import (
+    compute_vpin,
+    compute_vpin_time_series,
+    fill_toxicity,
+    adverse_selection_ratio,
+    liquidation_clustering,
+)
+
+from microstructure.funding import (
+    funding_regime,
+    funding_predictability,
+    funding_carry_pnl,
+    basis_spread,
+    basis_convergence_speed,
+)
+
+from microstructure.signals import (
+    composite_signal,
+    SignalPipeline,
+    detect_hft_regime,
+)
+
+__all__ = [
+    "microprice", "mid_price", "order_book_imbalance", "depth_imbalance",
+    "spread_stats", "depth_resiliency", "queue_depletion_prob", "batch_book_stats",
+    "classify_lee_ready", "classify_bulk_lee_ready", "compute_markouts",
+    "markout_summary", "trade_volume_profile", "trade_arrival_rate",
+    "compute_vpin", "compute_vpin_time_series", "fill_toxicity",
+    "adverse_selection_ratio", "liquidation_clustering",
+    "funding_regime", "funding_predictability", "funding_carry_pnl",
+    "basis_spread", "basis_convergence_speed",
+    "composite_signal", "SignalPipeline", "detect_hft_regime",
+]