feat: add instrument-aware fee schedules

[fallblu]

Summary

• add deterministic, composable per-instrument fee schedules with named fixed, notional, and per-unit components
• support maker/taker applicability, explicit rounding, per-fill minimums and caps, rebates, and multi-currency FX conversion
• attribute fee components through fills, positions, valuations, journals, and strategy messages while preserving frozen v8/v6 contracts
• publish scenario/journal contract v9 and strategy protocol v7 with regenerated fixtures and documentation

Refs #27

Test plan

• PATH="$PWD/.venv-schema/bin:$PWD/.venv-docs/bin:$PATH" make check
• PATH="$PWD/.venv-schema/bin:$PWD/.venv-docs/bin:$PATH" make coverage
• .venv-schema/bin/python test/validate_contract_conformance.py