refactor: Produce native relationship observations (#13500)
## Summary
- Add a public parser-neutral relationship vocabulary and retain an
immutable relationship generation validated against repository
knowledge.
- Classify compatibility descriptors once, then preserve existing
package graph edges and unresolved external declarations as projections.
- Emit Cargo internal relationships natively without synthetic
JavaScript dependency maps.
- Preserve complete declarations for later projections while keeping
current first-wins and peer-exclusion graph behavior.
## Stack
- Depends on #13499.
- Linear:
[TURBO-5801](https://linear.app/vercel/issue/TURBO-5801/produce-native-package-relationship-observations)
## Testing
- `cargo test -p turborepo-repository`
- `cargo clippy -p turborepo-repository --all-targets -- -D warnings`
- `cargo test -p turbo --test relationship_projection_contract`
- Focused Cargo task graph, affectedness, dependency invalidation, and
prune tests
- `cargo fmt --all -- --check`
- `git diff --check`
