feat: replace cuda13 flavour with per-GPU-generation packages

Build separate packages for each GPU generation instead of a single
cuda13 package:
- mistralrs-ampere (sm_86, RTX 3060)
- mistralrs-ada (sm_89, RTX 4090)
- mistralrs-blackwell (sm_120, RTX 5090)

All use the same CUDA 13.0 toolkit and features (cuda, cudnn, flash-attn,
nccl), varying only the compute capability target.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

rpm/mistralrs.spec

@@ -4,7 +4,7 @@
 
 # Passed in via --define at rpmbuild time
 %{!?mistralrs_version: %global mistralrs_version 0.7.0}
-%{!?mistralrs_flavour: %global mistralrs_flavour cuda13}
+%{!?mistralrs_flavour: %global mistralrs_flavour blackwell}
 
 # For prerelease builds, pass --define "mistralrs_prerelease 0.1.YYYYMMDDgitSHORTSHA"
 %if 0%{?mistralrs_prerelease:1}