feat: add causal patch inbox tracer

[developer-1px]

Summary

• add a private causal JSON Patch inbox lab for dependency-declared, out-of-order delivery and exact deduplication
• apply each currently ready envelope as one document patch, advancing the causal frontier only after success
• expose explicit blocked, faulted, diverged, and disposed states without changing the core public API
• use a reverse dependency index, min-heap ready queue, stack-safe incremental cycle checks, and defensive JSON ownership
• document performance bounds, non-goals, friction, and the next ready-time materialization experiment

Validation

• npm run verify
• npm run standard:check
• 25 causal inbox interface tests
• 41/41 extension labs verified
• three independent reviews found no remaining blocker
• git diff --check