refactor: Migrate native task definition precedence (#13532)
## Intent
Continue Phase 4: the engine task-definition resolver must decide
command precedence using **catalog authorship / registration /
executability**, not by dispatching through behavioral `Toolchain`
callbacks.
Precedence (highest → lowest) stays: package command, root
`package#task` command, authored native command, unscoped/per-provider
command, synthesized native command.
**Stack:** Phase 4 layer 5. Base =
`shew/turbo-5819-migrate-persistent-and-recursive-task-validation`.
## Changes
- `defines` / `registers` / `authors` for definition resolution read the
native-task catalog
- Remove toolchain callback dispatch from those precedence checks
## Out of scope
Execution framing, task contracts / derived I/O (TURBO-5789).
## Testing
- `cargo test -p turborepo-engine --lib` (127 passed)
- `cargo clippy -p turborepo-engine --all-targets -- -D warnings`
Closes TURBO-5820.
Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Anthony Shew <anthonyshew@users.noreply.github.com>
