Fix two compilation errors at lines 584 and 658 where code was calling
.code on &String diagnostics. Replaced d.code.to_string() with direct
Vec<String> clone since diagnostics is already Vec<String>.
Accepts criteria:
- cargo check -p pdftract-cli emits no 'no field code' errors
- serve.rs compiles cleanly
- Add explicit type annotation to migrations HashMap
- Box the identity closure to match Box<dyn Fn> signature
- All 9 unit tests pass
- CLI identity migration and error handling verified
Verification: notes/pdftract-1wy98.md