Support three-digit CUDA compute architectures

[Copilot]

fixes #21

parse_and_validate_compute in build.rs rejected any compute capability with more than 2 digits, breaking builds for GPUs like the RTX 5090 (Blackwell, sm_120) when setting HYPERBEAM_CUDA_COMPUTE=120.

Changes

• build.rs: Relax validation from len() != 2 to len() < 2 || len() > 3, accepting both 2- and 3-digit compute values (e.g. 86, 120)
• Update panic message and comment to reflect 2- or 3-digit constraint

# Previously panicked; now works
HYPERBEAM_CUDA_COMPUTE=120 cargo build --features=cuda,gpu-single

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.70%. Comparing base (dd61c6e) to head (fac9095).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   84.06%   84.70%   +0.64%     
==========================================
  Files          14       10       -4     
  Lines        2611     1563    -1048     
==========================================
- Hits         2195     1324     -871     
+ Misses        416      239     -177     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.