PTX backend.
Lowers Rust meta-HIR (functions annotated #[gpu_kernel(...)]) to
NVIDIA PTX text targeting sm_80+. Layer 5 compile contract:
contracts/compile-rust-to-ptx-mma-v1.yaml.
Architecture (PMAT-264 / Section 29): [PtxBackend] wraps a
[MultiEmitterBackend] so emission routes through the same
general/specialist quorum framework that will eventually carry
rustc_codegen_nvvm (general) + aprender-gpu (specialist). At
v0.1.0 the wrapper holds a single [ScaffoldPtxEmitter] in the
general slot — the same code path real emitters will plug into.
When rustc_codegen_nvvm lights up (next phase per
sub/layer5-multi-emitter-quorum.md), it slots into the general
position; when aprender-gpu ships its bridge, it slots into the
specialist position; no changes to [PtxBackend]'s public API.