Boot-time validator for TRACK-03 configuration (D-32).

When ANY loaded mailable module has tracking: [opens: true] or tracking: [clicks: true] in its @mailglass_opts, the config :mailglass, :tracking, host: key is REQUIRED. Boot fails with %Mailglass.ConfigError{type: :tracking_host_missing} on omission.

Adopter usage (v0.1)

Call from your Application.start/2 callback after Mailglass.Config.validate_at_boot!/0:

def start(_type, _args) do
  Mailglass.Config.validate_at_boot!()
  Mailglass.Tracking.ConfigValidator.validate_at_boot!()
  # ...
end

v0.5 will auto-wire this into Mailglass.Config.validate_at_boot!/0.

Detection algorithm

Walks :code.all_loaded/0 checking for modules that export both __mailglass_mailable__/0 (discovery marker) and __mailglass_opts__/0 (compile-time opts reflection). Any module with opens: true or clicks: true in its :tracking opts triggers the host check.