[x509] native wn_x509 lightweight X.509 parser as default cert backend

[aidangarske]

Summary

Adds wn_x509, a hand-written zero-allocation X.509 v3 parser + verifier as an
opt-in cert backend for the TLS 1.3 client (WOLFNANO_X509_LITE, make X509_LITE=1).
The default stays wolfSSL asn.c/DecodedCert (full DN, policies, name
constraints, CRL/OCSP, ML-DSA leaf keys). The cert path calls one stable
wn_X509_* seam; the backend is a compile-time choice.

Provenance: the DER TLV layer is lifted from wolfTPM tpm2_asn.c; the field walk
follows wolfSSL examples/asn1/asn1.c + RFC 5280. All signature math stays in
wolfCrypt wc_*. In-place references into the caller's DER, no allocation.

Footprint (Cortex-M33, -Os -flto -Wl,--gc-sections, ArmGNU 14.2)

TLS 1.3 cert client	WOLFNANO_X509_LITE (native)	default (wolfSSL asn.c)	reduction
X.509, P-256	53.1 KB (54,396 B)	62.4 KB (63,877 B)	14.8%
X.509 + ML-DSA-44	67.8 KB (69,396 B)	79.8 KB (81,722 B)	15.1%

The LITE backend drops asn.c's reachable cert parser (~14-16 KB after
gc-sections) for the ~4-5 KB wn_x509 walker. Reproduce: sh bench/footprint-clients.sh.

Parser + verify (feature-macro gated)

• Fields: SPKI/raw pubkey, keyAlg/curve/sigAlg with an inner==outer sigAlg match,
  validity, subject CN; strict DER (minimal lengths, exact TLV consumption).
• Extensions: basicConstraints, keyUsage (padding bits checked), extKeyUsage, SAN
  dNSName; critical-unknown fail-closed, duplicate-ext reject, dNSName
  embedded-NUL reject.
• Chain: per-hop signature (ECDSA/RSA/Ed25519), keyCertSign + CA enforced on
  presented intermediates, pinned anchor trusted a priori; pure-function
  wn_X509_TimeValid (caller supplies the time via the XTIME seam).

Intended scope (use the default asn.c backend for these): no DN name-chaining,
pathLen, name constraints, or CRL/OCSP; the model is a pinned trust anchor.

Testing (both backends, vs wolfSSL master + pin)

• Differential vs wc_ParseCert over 13 certs (ECC P-256/P-384, Ed25519,
  RSA-2048/3072), every field byte-compared.
• Negative suite: crafted adversarial certs + all 9 wolfSSL certs/test/bad-*
  vectors; chain-verify, generated-PKI constraint tests, ML-DSA-44 leaf, libFuzzer harness.
• Hardened through multiple skoll RFC 5280 / security passes and independent code review.