fix(execution): bridge cash-only sell-funded buys

[Pigbibi]

Summary

• bridge projected sell proceeds into live cash-only buy budget when cash snapshots are stale
• allow a 1-share top-up for existing underweight holdings that miss by less than one whole share
• add a regression test covering the SOXL sell / SOXX buy live cash-only path

Testing

• python3 -m pytest -q tests/test_execution_service.py tests/test_runtime_broker_adapters.py

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