feat(labs): materialize causal intents when ready

[developer-1px]

Summary

• compose the causal inbox with the existing positional and stable-id rebase labs
• add a closed authored-intent union while preserving direct envelope behavior and result shapes
• materialize only after dependencies are ready, then advance causal state after one successful document commit
• retain structured policy failures, selection diagnostics, and defensive authored-payload deduplication
• teach the lab evaluator exact private-lab dependencies, changed-scope closure, cycle rejection, and dependency-level verification

Boundary

This is not a CRDT, OT, transport, or convergence protocol. Positional replay still requires an authored base and ordered gap-free concurrent batches; when the causal DAG cannot prove that relationship, planner conflicts and final guards prevent unsafe overwrite but may conservatively block a valid edit.

Verification

• causal inbox package: typecheck, 34 tests, build, runtime import
• all 41 extension labs: dependency-level typecheck, test, build, runtime import
• core: typecheck, 457 tests passing with 4 skipped, build, package smoke
• all official extensions: check, typecheck, test, build
• standard conformance and generated docs evaluation