Reuse shared native policy parsing in mxc_ffi

### Description of the task

Refactor the C ABI policy path in `src/ffi/mxc_ffi/src/lib.rs` to reuse the shared native policy parser and telemetry-version validation instead of maintaining the parallel `FfiPolicyEnvelope` / `validate_canonical_telemetry_version` implementation.

Preserve the FFI's typed error-code and error-detail behavior while ensuring policy JSON is interpreted once by the same contract used by the native execution boundary.

### Additional context

Identified while reviewing PR #821. Issue #1085 tracks the related but separate state-aware Node/.NET version-policy duplication; this task is scoped to the Rust C ABI/FFI parsing boundary.

Start in src/ffi/mxc_ffi/src/lib.rs and compare the C ABI policy path with the shared native policy parser and telemetry-version validation used by the native execution boundary. Review PR #821 for context; done means the parallel FfiPolicyEnvelope and validate_canonical_telemetry_version logic is reused while typed FFI error codes and details remain unchanged.

Written by the indexing model from the issue text.