perf: Reuse Cargo metadata discovery snapshot (#13600)
## Summary
- Use one `cargo metadata --locked --all-features` snapshot for
successful Cargo workspace discovery and local-package validation.
- Filter full metadata through Cargo's `workspace_members` and reuse the
validated lockfile bytes for pruning and external dependency closures.
- Preserve `--no-deps` as the fallback for memberless workspaces and
existing error precedence.
- Update the Cargo architecture description and add regression coverage
for external packages and missing-lockfile precedence.
## Tracking
Part of TURBO-5799, specifically the final verification requirement to
remove duplicated expensive observations. A focused Linear child issue
could not be created because Linear authentication is currently
unavailable.
## Testing
- `cargo fmt --all -- --check`
- `cargo test -p turborepo-repository`
- `cargo clippy -p turborepo-repository --all-targets -- -D warnings`
- Pre-push hooks
