feat(security): ES256 and Ed25519 JWT signing/verification (closes #401)

[gg582]

Summary

Closes #401. JWT gains asymmetric algorithms alongside the unchanged HS256 entry points:

• cwist_jwt_sign_es256 / cwist_jwt_verify_es256 — ECDSA over P-256 with SHA-256; JOSE raw R||S signatures (RFC 7518 §3.4); optional kid header claim
• cwist_jwt_sign_ed25519 / cwist_jwt_verify_ed25519 — EdDSA (RFC 8037)

Keys are PEM (SEC 1 or PKCS#8). Verifiers accept a public key or the public half of a private key.

Security properties

• Alg confusion closed: each verifier requires the header to declare exactly its own alg, so HS256/ES256/EdDSA tokens never cross-verify (RFC 8725 §3.1).
• Key type pinned: ES256 rejects any non-P-256 key (wrong curve or non-EC), Ed25519 rejects any non-Ed25519 key — signing and verification both fail cleanly on garbage PEM.
• HS256 keeps its constant-time compare; existing cwist_jwt_sign/cwist_jwt_verify signatures and behavior are unchanged (the bodies are folded into shared helpers).

Implementation notes

• ES256 converts between ASN.1 DER (BoringSSL EVP) and JOSE raw R||S with ECDSA_SIG_from_bytes/ECDSA_SIG_to_bytes + BN_bn2bin_padded.
• Public-key loading falls back from PEM_read_bio_PUBKEY to PEM_read_bio_PrivateKey with a BIO_reset (a failed first read consumes the buffer).
• exp/nbf validation with the existing 300 s leeway applies to all algorithms.

Tests

• tests/test_jwt_asym.c (new, in TEST_TARGETS): ES256/Ed25519 round trips, kid header, wrong-key, wrong-curve, wrong-type, garbage-PEM, expired-token, and full cross-algorithm confusion matrix.
• test_jwt and test_middleware_jwt pass unmodified (HS256 regression check).
• Rust bindings: sign_es256/verify_es256/sign_ed25519/verify_ed25519 in cwist::jwt with two new tests in tests/advanced.rs (14/14 pass).
• docs/api/v4.0-api-baseline.txt regenerated (make api-baseline; make api-check green).

[gg582]

make[1]: Entering directory '/home/runner/work/CWIST/CWIST/example/jwt/step-1-sign-verify'
cc -fsanitize=address,undefined -Wall -Wextra -g -I../../../include -I../../../lib -I../../../lib/libttak/include main.c -L../../../ -L../../../lib/cjson -L../../../lib/libttak/lib -lcwist -lcjson -lssl -lcrypto -lttak -lpthread -ldl -lstdc++ -lm -lz -o step-1-sign-verify
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_sign_bytes':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:231:(.text+0x4701): undefined reference to `EVP_PKEY_id'
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_key_is_p256':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:200:(.text+0x5002): undefined reference to `EVP_PKEY_id'
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_sign_bytes':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:264:(.text+0x50c4): undefined reference to `ECDSA_SIG_from_bytes'
/usr/bin/ld: /home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:273:(.text+0x5138): undefined reference to `BN_bn2bin_padded'
/usr/bin/ld: /home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:274:(.text+0x518a): undefined reference to `BN_bn2bin_padded'
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_verify_bytes':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:320:(.text+0x5ddc): undefined reference to `EVP_PKEY_id'
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_key_is_p256':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:200:(.text+0x5e29): undefined reference to `EVP_PKEY_id'
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_load_public_key':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:191:(.text+0x640c): undefined reference to `BIO_reset'
/usr/bin/ld: ../../..//libcwist.a(jwt.o): in function `jwt_verify_bytes':
/home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:343:(.text+0x681a): undefined reference to `ECDSA_SIG_to_bytes'
/usr/bin/ld: /home/runner/work/CWIST/CWIST/src/security/jwt/jwt.c:345:(.text+0x6b9e): undefined reference to `OPENSSL_free'
collect2: error: ld returned 1 exit status

Build fails