feat(ffi): complete FFI bridge for all 48 prover backends

- Expand kind_from_u8/kind_to_u8 from 13 to 49 prover mappings
- Add C-compatible API (echidna_init, echidna_create_prover,
  echidna_parse_string, echidna_verify_proof, echidna_apply_tactic,
  echidna_suggest_tactics, echidna_export_proof, echidna_last_error,
  echidna_version) matching interface wrapper declarations
- Add roundtrip test verifying all 49 prover kind mappings
- Add tests for C API functions
- 9/9 FFI tests pass

This closes the v2.0 roadmap item "FFI/IPC bridge for interfaces → Rust core"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath