refactor(rust): improve graph schema caching and CAS boundaries

Summary

• Cache built-in target kind schemas with OnceLock and reuse one schema set across batch graph construction.
• Add a regression test that verifies built-in schema enrichment for multiple targets.
• Move once-cas public errors and cache result/stat models into focused modules while preserving existing re-exports.

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test --workspace
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo build --release
• mise exec -- shellspec (147 examples, 0 failures, 1 expected skip)

No GitHub comments yet.