Adds `_internal_test.go` files to test unexported functions in the `crypt` and `lthn` packages, improving overall test coverage.
- Adds tests for the `ensureRSA` private function in `pkg/crypt/crypt.go`.
- Adds tests for the `createSalt` private function in `pkg/crypt/std/lthn/lthn.go`.
These changes align with the project's TDD methodology and follow the `_Good`, `_Bad`, `_Ugly` testing structure.
This commit restructures the `crypt` service to be more modular and decoupled from storage concerns.
- The standard cryptographic implementations (`lthn`, `chachapoly`, `rsa`) have been moved to the `pkg/crypt/std` directory.
- The `rootfs` components have been removed to decouple the library from storage.
- Import paths have been updated to reflect the new structure.
