feat: stage Longbridge paper execution command producer

[Pigbibi]

Summary

• advance the QuantPlatformKit pin to the compatible durable-command contract
• allow an opt-in paper-only Longbridge cycle to enqueue one immutable, content-addressed command
• expose a sanitized queued or already-queued observation without any claim, broker routing, or fill state
• reject configuration that tries to enable this producer outside dry-run mode

Safety

• no deployed environment enables the flag or configures a command store
• the Longbridge SG live target remains disabled
• this does not add a command consumer, retry path, broker lookup, reconciliation action, or live authority

Verification

• 60 focused tests passed
• uv lock check passed
• project ruff check passed

Tracks #359 and QuantStrategyLab/QuantPlatformKit#342.