[upstream-sync] fix(engine): submit parent target states after child readiness (upstream #2014)

[github-actions]

Upstream Change Summary

Type: bug-fix
Difficulty: Medium
Recommendation: Adopt

Parent component target states were submitted before child components signaled readiness, creating a race where a parent's persisted state could appear consistent while its children had not yet finished initializing. The fix delays the parent's submit until child readiness is confirmed and keeps component submit serialized under the build semaphore. A regression test test_parent_sink_ordering was added to verify the ordering invariant is maintained.

Upstream References

• PR: fix(engine): submit after child readiness cocoindex-io/cocoindex#2014
• Upstream repo: https://github.com/cocoindex-io/cocoindex

Relevant Upstream Files / Areas

• rust/cocoindex/src/engine/ — component build logic, submit path, build semaphore

Recoco Considerations

• Affected files: crates/recoco-core/src/engine/ (component execution, build semaphore management)
• No Python code to exclude; no feature-gating required
• The regression test should be ported alongside the fix — it validates a correctness property that's easy to accidentally regress
• The build semaphore scoping change needs careful integration with recoco's existing component lifecycle to avoid introducing new deadlocks

Integration Notes

This is a correctness fix for nested component scenarios. Identify the equivalent submit/readiness coordination in recoco's engine. The upstream fix involves awaiting a child-readiness signal before the parent's submit() call while keeping the build semaphore held. The test case test_parent_sink_ordering from the upstream PR provides a regression harness that should be adapted to recoco's test infrastructure.