strategy: implement Strategy ABC and signal framework

[marwinsteiner]

Summary

Phase 3 Round 1: Strategy ABC and Signal Framework implementation.

• src/sysls/strategy/base.py -- Strategy ABC (main user extension point) and StrategyContext

  • StrategyContext: provides bus/clock access plus emit_signal() and request_order() convenience methods
  • Strategy ABC: abstract on_market_data(), optional lifecycle hooks (on_start, on_stop, on_fill, on_position), convenience wrappers for signal emission and order requests
  • Constructor injection via StrategyContext (no global state)

• src/sysls/strategy/signal.py -- Signal model, SignalBook, and combinators

  • Signal: frozen Pydantic model with direction, strength (clamped to [-1.0, 1.0] via model_validator), instrument, strategy_id, timestamp_ns, metadata
  • SignalBook: mutable container tracking latest signal per instrument with optional max_age_seconds for stale signal filtering
  • Combinators: combine_signals_average, combine_signals_majority, combine_signals_weighted
  • Conversion utilities: signal_from_event / signal_to_event for bus event integration

• 50 tests total (18 base + 32 signal), all passing

• Full ruff lint + format compliance

• Zero regressions across the full 472-test suite

Test plan

• StrategyContext properties (bus, clock accessible)
• StrategyContext emit_signal publishes SignalEvent on bus
• StrategyContext request_order creates OrderRequest and publishes OrderSubmitted
• Strategy init stores attributes correctly
• Default params is empty dict
• Lifecycle hooks (on_start, on_stop, on_fill, on_position) are no-ops by default
• Cannot instantiate Strategy without implementing on_market_data (ABC enforcement)
• Convenience methods delegate to context correctly
• Signal model creation, strength clamping, immutability
• SignalBook CRUD (update, get, remove, clear, contains, len)
• SignalBook stale signal filtering with max_age_seconds
• All three combinator functions (average, majority, weighted)
• Signal/SignalEvent round-trip conversion
• Full test suite regression check (472 passed, 10 skipped)