feat(x509): verify certificate chains (linkage / ordering / expiry)

[xunholy]

Summary

Extends the X.509 PKI toolset from decoding into verification — the #1 real-world PKI debugging task ("the chain is present but not trusted"). Complements x509_certificate_decode / csr_decode / crl_decode / ocsp_decode.

x509_chain_verify (Low risk, offline, read-only) takes a PEM bundle (leaf→root order) or a hex-DER concatenation and checks the signature linkage between adjacent certificates via crypto/x509.CheckSignatureFrom (the parent must cryptographically sign the child and be a CA permitted to sign certificates). Reports:

• ordered — every adjacent link verifies in the supplied order;
• reaches_self_signed_root — a self-signed trust-anchor terminates the chain;
• any_expired — any cert past NotAfter (a common "looks right but rejected" cause; reported, not folded into link validity);
• per-cert subject / issuer / self-issued / is_ca / expiry, and per-link validity with the exact error on a break.

Diagnoses the three usual failures: wrong ordering, missing intermediate, expired link.

Design / scope

• Linkage is signature + CA-constraint only (CheckSignatureFrom), not full RFC 5280 path validation against a trust store — stated plainly in the tool description. Expiry is surfaced per-cert but deliberately not folded into link validity (so a chain that links correctly but has an expired member is diagnosable as exactly that).
• internal/x509decode.VerifyChain parses every CERTIFICATE block (PEM) or DER concatenation. No new dependency.

Verification

• Tests build a real root → intermediate → leaf chain and cover ordered-to-root, wrong-order (links don't verify), single-cert, expired-link (linkage still valid), and bad-input cases; tool handler + discoverability tests; registry 701→702 + risk coverage.
• task ci green (lint 0 / vet / build / test:full 0 fail / govulncheck clean); task eval 17/17.

Feature — anchors v0.772.0.