feat: Compose aggregate native task dependencies (#13667)
### Description
Part 4 of 13 in stack #13677, based on #13666. Composes aggregate
children into engine task dependencies with effective-ID deduplication,
memoization safety, normal cycle validation, and qualified pass-through
guidance.
Previous: #13666. Next: #13668 completes override and exclusion
semantics.
### Testing Instructions
Full-tip verification passed: formatting; 437 repository tests; 135
engine tests; 13 uv integration tests; 12 entrypoint tests; and strict
clippy across affected crates. Every cumulative tip passed `cargo check
--tests`. `uv` is unavailable, and a full workspace run was not
performed.
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
