[nihalpasham]

Summary

This adds the first useful slice of device debug info: line tables. It carries rustc spans into pliron locations, emits the minimal LLVM debug metadata needed by NVPTX, and wires the debug gate through rustc -C debuginfo plus a CUDA_OXIDE_DEBUG override.

Concretely:

• decode MIR spans into Location::SrcPos instead of stringifying them
• add a shared metadata ID allocator for NVVM + debug metadata
• emit DIFile, DICompileUnit, DISubprogram, DILocation, !llvm.dbg.cu, and debug module flags
• attach !dbg to function definitions and real LLVM instructions
• keep constants, virtual ops, synthesized PHIs, and variable/type debug info out of this first pass
• keep missing call spans valid for LLVM inlining by falling back to the containing function line

Closes #224.
Closes #105.

Follow-up for full variable/type debug metadata: #226.
Related: #106, #103, #67.

Validation

• cargo fmt --all -- --check
• cargo fmt --all -- --check in crates/rustc-codegen-cuda
• cargo clippy --workspace -- -D warnings
• cargo clippy --all-targets -- -D warnings in crates/rustc-codegen-cuda
• non-example unit test matrix
• cargo test -p cuda-core --lib
• cargo test --manifest-path crates/rustc-codegen-cuda/Cargo.toml
• cargo test -p llvm-export
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --workspace
• cargo test --doc --workspace --exclude cuda-bindings
• scripts/check-error-example-status.sh
• naming guard equivalent
• cargo deny check
• CUDA_OXIDE_DEBUG=line-tables cargo oxide pipeline compiler_features
• direct opt-20 -O2 round-trip on compiler_features.ll
• manual cuda-gdb smoke: vecadd hits device source lines and shows a device backtrace through cuda-device/src/thread.rs and src/main.rs

Full examples smoketest is being run separately.

[nihalpasham]

Follow-up from testing a larger example: CUDA_OXIDE_DEBUG=line-tables cargo oxide pipeline compiler_features exposed one LLVM verifier issue. Some inlinable calls had a debug scope through the function, but the call instruction itself had no !dbg, so opt treated the debug info as invalid and stripped it before PTX.

I pushed a narrow fix in 72c8ed6: calls without their own source span now fall back to the containing function line. Other synthetic ops still stay location-less, so we avoid inventing extra stepping stops. Verified with cargo test -p llvm-export, direct opt-20 -O2, and the compiler_features pipeline; optimized IR keeps !llvm.dbg.cu/DILocation, and PTX now has .target ..., debug, .file, and .loc.