Fix cross-platform release build (Linux/Windows)

[neuromechanist]

Summary

Fixes the release matrix (#167) so the native binaries actually build on Linux and Windows. Validated by dispatching the workflow on this branch: macos-arm64, linux-x64, linux-arm64 and windows-x64 all build and smoke-test green (4 artifacts uploaded). windows-arm64 remains experimental (no MSYS2 gfortran for CLANGARM64; tracked in #173).

Fixes

• Linux: -l:lblas.a -> -l:libblas.a (the -l: syntax needs the full archive filename; Ubuntu ships libblas.a).
• Windows (MSYS2 MINGW64): build.sh used python3 and cc, which the mingw shell does not provide; build.sh now falls back to python/gcc, and the workflow installs mingw-w64-*-python.

Test plan

Dispatched run: 4/5 native targets green, each smoke-tested (2-iteration fit on real sample EEG -> finite negative LL). windows-arm64 fails at toolchain setup only (continue-on-error), tracked in #173.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.62%. Comparing base (c0a5cc3) to head (9acf6b7).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   82.95%   82.62%   -0.33%     
==========================================
  Files          19       19              
  Lines        2435     2435              
  Branches      412      412              
==========================================
- Hits         2020     2012       -8     
- Misses        299      308       +9     
+ Partials      116      115       -1     

see 1 file with indirect coverage changes

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