feat: observe paper command gate [skip-cloud-run-deploy]

[Pigbibi]

Scope

• wire the shared runtime command gate into the existing LongBridge paper-only durable command producer
• persist its observation receipt in the execution report
• advance the QPK pin to the already-merged command-gate contract

Safety

• command production remains paper-only and create-only
• the consumer remains explicitly unauthorized
• gate enforcement remains observe
• target-level commands report unknown net exposure and therefore would be blocked from enforced broker routing until a future consumer reconciles per-order positions
• this PR carries [skip-cloud-run-deploy]; it must not build, deploy, sync environment, or change traffic

Verification

• uv run --frozen --no-sync --extra test pytest tests/test_durable_execution_commands.py tests/test_runtime_composer.py tests/test_request_handling.py -q (30 passed)
• uv run --frozen --no-sync --extra test pytest tests/test_rebalance_service.py -q -k records_execution_marker_after_dry_run_order_preview_or_blocks_live_next_session_decision_without_routing (2 passed)
• uv run --frozen --no-sync --extra test ruff check relevant source and tests (passed)