Backend trait — code-lane emission abstraction.
Every target language in xpile (Rust, Ruchy, PTX, WGSL, SPIR-V, Lean)
provides one type implementing [Backend]. The trait is intentionally
narrow: take meta-HIR and a config, return an [Artifact].
Sibling of xpile-frontend::Frontend. Architectural invariants
codified in contracts/xpile-backend-trait-v1.yaml.