Add standard signal snapshots

[Pigbibi]

Summary

• add signal_snapshot.v1 payload builder
• attach signal snapshots to IBKR notifications, runtime logs, and execution reports
• carry execution price-source fallback information into the snapshot when available

Tests

• .venv/bin/python -m pytest tests/test_rebalance_service.py tests/test_request_handling.py -q
• python -m py_compile application/signal_snapshot.py application/rebalance_service.py notifications/renderers.py main.py

Note: local worktree had unrelated uncommitted IBKR WIP; this PR only stages the signal snapshot files listed above.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Refresh snapshot after execution pricing fallback

When an executed cycle later uses execute_rebalance price fallbacks, this pre-execution snapshot is the one rendered into dashboard/strategy_dashboard and then passed to render_trade_notification, and it is never refreshed after execution_summary supplies price_source_mode or snapshot_price_fallback_used. In that scenario the notification’s signal snapshot still reports ibkr_strategy_market_data with no fallback warning even though the runtime report is corrected later in main.py; update signal_metadata['signal_snapshot'] from the execution summary before publishing the trade notification.

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