refactor: migrate crypto-refresh EC/Ed code from Botan native API to FFI

[ronaldtse]

Summary

Addresses the Botan 3.12 ENABLE_CRYPTO_REFRESH build breakage by migrating the crypto-refresh EC/Ed code off Botan's unstable native C++ API and onto the stable FFI — the same discipline the rest of rnp already follows (which is why it rides across Botan 2.x/3.x untouched).

The native-API code bit-rotted on 3.12: tightened include hygiene made Botan::EC_Group / EC_AffinePoint / BigInt incomplete types, and public_point() + the Ed25519_PrivateKey(span) ctor are deprecated.

Migrated files (now contain zero Botan:: references):

• src/lib/crypto/ec.cpp — ec_generate_generic_native mirrors the existing FFI keygen in Key::generate() (botan_privkey_create + get_field); native bigint.h/ecdh.h includes dropped.
• src/lib/crypto/ed25519_ed448.cpp — Ed25519/Ed448 keygen, sign, verify, validate via botan_privkey_create/load_*, botan_pk_op_sign_*/verify_*, botan_*_check_key.
• src/lib/crypto/exdsa_ecdhkem.{cpp,h} — replaced the Botan::* key-object helpers with FFI key handles; ECDH KEM via botan_pk_op_key_agreement_*, ECDSA sign/verify, check_key, SEC1 loaders. Private key_ member moved from Botan::secure_vector to rnp::secure_bytes; all <botan/*> includes removed from the header.

FFI gap audit (for discussion with Jack Lloyd)

TODO.refactor/botan-ffi-migration.md documents a full audit: no Botan FFI gaps were found. Every native op rnp uses (ECDH/ECDSA/Ed25519/Ed448/X25519/X448 keygen+load, key agreement, KEM, sign/verify, check_key, ML-DSA/ML-KEM/SLH-DSA loaders, RFC 3394 keywrap) has a stable FFI equivalent. Nothing needs to be requested upstream. The doc also lays out the Phase 2 roadmap (migrate the remaining native-API files: x25519_x448.cpp, dilithium*, kyber*, sphincsplus.cpp).

Test plan

• cmake -DCRYPTO_BACKEND=botan3 -DENABLE_CRYPTO_REFRESH=ON builds clean on Botan 3.12 (native-API errors gone).
• Full rnp_tests suite passes: 280 tests (v6 SKESK/sig/import, ECDSA keygen, Ed25519/Ed448, ECDH, CLI examples).
• All four migrated files contain zero Botan:: references.

Notes / follow-ups

• ed25519_ed448.cpp still calls the deprecated botan_privkey_ed25519_get_privkey / botan_*_ed448_get_* getters (consistent with the existing eddsa.cpp on main). These are the same -Wdeprecated-declarations class that PR fix: silence -Wnonnull-compare and Botan FFI deprecation warnings #2446 migrates to botan_*_view_raw; that migration should be extended to these three new call sites (tracked in the roadmap doc).
• Phase 2 (the remaining native-API files) is intentionally out of scope for this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.38%. Comparing base (975b67c) to head (a77b894).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2447   +/-   ##
=======================================
  Coverage   85.37%   85.38%           
=======================================
  Files         126      126           
  Lines       22790    22787    -3     
=======================================
- Hits        19458    19456    -2     
+ Misses       3332     3331    -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ronaldtse]

Consolidating this into #2448, which rebases the same FFI migration on top of kaie's pqc-openssl (PR #2392). That's the better base because #2392 restructures exdsa_ecdhkem.{cpp,h} (backend split), which conflicts with the exdsa change carried here.

#2448 completes the full migration (all crypto-refresh/PQC Botan-side files, not just Phase 1), fixes the Botan 3.12 build on kaie's branch too, and passes 296/296 tests including all PQC tests. Closing this in favor of #2448.