ae.net.ssl.schannel: Add Windows SChannel TLS backend

Implements the full SChannel/SSPI-based TLS backend for ae.net.ssl,
matching the cross-platform SSLContext API introduced in previous commits.

Key design decisions and technical notes:
- Entire module body guarded by `version (Windows):` so the file compiles
  to an empty translation unit on Linux/macOS with no changes needed.
- Uses SCH_CREDENTIALS (Win10 1809+) with TLS_PARAMETERS deny-list for
  protocol version control, instead of legacy SCHANNEL_CRED.
- PFX import uses the "perphemeral" pattern (dwFlags=0 + key container
  deletion on teardown via NCryptDeleteKey / CryptAcquireContextW), mirroring
  .NET's SafeCertContextHandleWithKeyContainerDeletion.  PKCS12_NO_PERSIST_KEY
  is intentionally not used: LSASS cannot dereference an in-process
  NCRYPT_KEY_HANDLE across the process boundary required for server credentials.
- DecryptMessage SEC_I_RENEGOTIATE handling is load-bearing for TLS 1.3:
  Windows sends two NewSessionTicket messages post-handshake, causing every
  TLS 1.3 server connection to fail if this path is not implemented.
- ApplyControlToken is redeclared locally without A/W suffix to work around
  a druntime bug (secur32.dll exports only the unsuffixed symbol).
- Includes bindings supplement for symbols missing from druntime 2.111:
  SP_PROT_TLS1_1/1.2/1.3, SCH_CREDENTIALS struct family, PFXImportCertStore,
  CertEnumCertificatesInStore, CertDuplicateCertificateContext,
  CertGetCertificateContextProperty, CryptStringToBinaryA, ncrypt functions.
- Convenience methods setCertificateContext() and setCredentialsFromSystemStore()
  for Windows certificate store integration (non-portable, SChannel-specific).
- Unittests: PFX echo server (pre-baked 2555-byte fixture, CN=localhost) and
  cross-backend corpus via testSSL(), both gated on debug(ae_unittest).

Author: Spike