Gateway for the optional Sigra integration ({:sigra, "~> 0.2"}).

Sigra provides distributed tracing primitives that mailglass hooks into for cross-boundary span propagation. When sigra is present, this module is compiled and available?/0 returns true. When absent, the module does not exist at all.

Unlike the other gateways (which always compile), the Sigra module is conditionally compiled because Sigra itself cannot be referenced at all — even in no_warn_undefined — in environments that use Sigra's own compile-time module discovery. Callers check existence via Code.ensure_loaded?(Mailglass.OptionalDeps.Sigra), not available?/0 directly.