Decoder-side audio backend traits for the labelle-toolkit. Defines a
comptime-validated Backend(Impl) wrapper and a DecodedAudio POD that
concrete decoder backends (raylib-audio, sokol-audio, miniaudio, …)
implement; labelle-assembler adapts the result to labelle-engine's
AudioBackend struct at codegen time. This is the audio sibling of
labelle-gfx's image and
font backend traits (see labelle-gfx#258). Runtime playback
(AudioInterface-style) lives in labelle-core and is intentionally not
part of this library. Tracking issue:
labelle-engine#530.