Harden PKIX path validation and support trusted certificate PEM blocks

[Xor-el]

Summary

Hardens certification path validation and building, and adds read/write support
for TRUSTED CERTIFICATE PEM blocks. Also tightens a few related surfaces
(trust-anchor comparison, ECDH KEK shared info).

Path validation and building

• Restrict CRL-signer validation to the supplied trust anchors and report the
  real failure per RFC 5280 sec. 6.3.3 (f). When the certificate's own
  distribution-point CRLs are tried first and fail, that context is chained into
  the final message instead of being lost.
• Count path length excluding self-issued intermediates, so a rolled root or an
  indirect CRL-signer chain is bounded correctly.
• Narrow the active parameters to the resolved trust anchor during validation,
  and raise when the resolved anchor is not among those supplied.
• Honour excluded certificates when building a path.
• Raise a recoverable error when no CRLs are found, rather than looping when two
  CAs share a CRL-issuer distinguished name.
• Find the trust anchor by comparing the CA and subject by value, preserving the
  first failure reason.

Trust anchor and ECDH KEK

• Compare the trust-anchor CA by value and drop the string CA-name form; callers
  and results now use CA.ToString().
• Include the optional entityUInfo in the ECDH KEK shared info.

OpenSSL PEM

• Read and write TRUSTED CERTIFICATE blocks (an X509 trusted certificate block
  wrapping the certificate plus an optional certificate trust block).

Tests

• Add CertPathHardeningTests covering the shared-CRL-issuer loop case and the
  indirect CRL-signer paths (single generation, rolled root, excluded signer,
  and maximum-path-length bounds).
• Add an OpenSSL trusted-certificate round-trip and an EdDSA private-key read,
  loading the key from the test data folder.
• Assert the trust-anchor CA round-trips via CA.ToString().