feat: add isolated Schwab paper command consumer

[Pigbibi]

Summary

• add an isolated, default-disabled /paper-command-consumer endpoint for Schwab
• use the shared durable command lifecycle with exact platform/account/strategy binding
• lazily read portfolio and quotes only after a command has passed that binding
• reconcile value targets, cash, total equity, managed symbols, shorts, and quote data before recording a paper proposal
• document the dedicated PAPER, disabled-runtime, cash-only, and dedicated-storage requirements

Verification

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=/tmp/qsl-qpk-paper-command-binding/src:. python -m pytest -q tests/test_paper_execution_command_consumer.py tests/test_paper_execution_admission.py
• PYTHONPATH=/tmp/qsl-qpk-paper-command-binding/src:. python -m compileall -q application/paper_execution_command_consumer.py main.py runtime_config_support.py tests/test_paper_execution_command_consumer.py
• git diff --check

The local full configuration test requires pandas, which is not installed on this constrained VPS; CI installs declared dependencies. No deployment, Cloud Run configuration, Scheduler, broker execution port, or live order path is changed.