0.6.4

No breaking changes.

• Fix: a corrupted or missing client certificate/key made SHCAPIAsync
  crash with a raw, cryptic ssl.SSLError: [SSL] PEM lib instead of a
  catchable error. This only happened on the fallback path (no pre-built
  ssl_context passed to the constructor — a direct library user, or a
  caller predating the ssl_context parameter); callers that pre-build the
  context off-loop (e.g. boschshc-hass) were already unaffected by this
  specific crash shape via their own error handling. SHCAPIAsync.__init__
  now catches ssl.SSLError/OSError/ValueError from build_ssl_context()
  and re-raises as SHCCertificateError, matching the existing convention
  already used by the sync-side certificate.py for the same failure class.
  Found while root-causing a community report of an integration setup
  crashing after a corrupted PEM file on disk.
• Hardening: register_client.py's write_tls_asset() now calls
  os.fsync() before closing the file handle. Without it, a crash or
  power-loss in the narrow window between a successful pairing and the OS
  flushing dirty pages to disk could leave a truncated/empty PEM file that
  only surfaces as a cryptic failure much later, on some unrelated future
  restart.