fix(cuda): make CUDARC_CUDA_VERSION a true version variable (drop hardcoded cuda-12080)

[isPANN]

Problem

The workspace pinned cudarc's cuda-12080 feature, which defeated the whole point of exposing the CUDA version via CUDARC_CUDA_VERSION. cudarc's build.rs does not let the env override a hardcoded feature — it emits an extra cargo:rustc-cfg=feature="cuda-<env>". So a hardcoded cuda-12080 plus a mismatched CUDARC_CUDA_VERSION=12060 activates both cfgs → cudarc compiles two binding sets → ~171 dup-definition errors (E0428, nvrtcResult variant-not-found). In practice we were pinned to 12.8.

Fix

Drop the hardcoded cuda-12080; use cuda-version-from-build-system + fallback-latest instead (no hardcoded version). Now the env is the sole selector:

• HPC: set CUDARC_CUDA_VERSION=<NNNNN> to match module load cuda/X.Y (env wins) — or leave it unset and nvcc auto-detects the loaded toolkit.
• macOS dev (no nvcc): fallback-latest picks cudarc's newest supported version purely for cargo check (dynamic-loading → never actually runs CUDA).

One-line change to [workspace.dependencies] cudarc features; Cargo.lock unaffected.

Verification

Build + runtime across cuda 12.0 / 12.2 / 12.4 / 12.6 / 12.8 on HKUST-GZ A40 — 56/56 tests each (Slurm job 9800420). Notably 12.6, which previously failed to even compile, now passes. macOS cargo check confirmed green via fallback-latest.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.29%. Comparing base (20e0aca) to head (2d5486c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files          19       19           
  Lines         918      918           
=======================================
  Hits          884      884           
  Misses         34       34           

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