feat: add fill feature logging for ML dataset building

[keitaj]

Summary

• Persist per-fill order book features (spread, book imbalance, micro-price skew, top-of-book sizes) plus the adverse selection tracker's 5s/30s/60s markout samples as one JSON line per fill, in a daily-rotated JSONL file — a ready-made supervised dataset for offline adverse-selection modelling.
• Feature definitions live in a single pure function (fill_features.compute_fill_features) shared by the logger and any future in-bot inference, structurally preventing train/serve skew.
• Observation-only and fail-silent: the WebSocket thread performs no file IO (in-memory buffer, main-loop flush after a 65s maturity window so markout labels land in the same line); IO errors, a daily size cap, and a buffer cap are all counted and never affect trading. Default OFF preserves existing behaviour exactly.

Changes

• fill_features.py (new): shared pure feature computation + FEATURE_SCHEMA_VERSION.
• ws/fill_feature_writer.py (new): FillFeatureWriter — buffered, maturity-gated, daily-rotated JSONL writer with size/buffer caps and rate-limited error logging.
• ws/adverse_selection_tracker.py: optional raw_fill kwarg on on_fill (backward compatible), feature record composition (tid/oid/hash join keys + features), set_feature_writer() hook, record field on FillSnapshot.
• ws/fill_feed.py: pass the raw userFills object to the tracker (raw_fill=fill).
• bot.py: writer wiring (warns and disables when the adverse tracker is unavailable), main-loop maybe_flush(), shutdown flush_all(), config keys + defaults, argparse flags.
• ws/__init__.py: export FillFeatureWriter.
• README.md: human-facing description + AI YAML parameter reference entries.
• Tests: test_fill_features.py, test_fill_feature_writer.py (new), plus additions to tracker/fill-feed tests and a stub-attribute fix in test_risk_check_throttle.py.

Test plan

• Unit tests added/updated (feature math, maturity gating, rotation, size/buffer caps, fail-silent paths, join-key passthrough, backward compatibility of legacy on_fill calls)
• flake8 passes
• pytest passes (1,149 tests)
• No regressions in existing tests (default OFF path unchanged)

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