Fix Windows pip packaging and cross-platform libHalide shadowing

[alexreinking]

Summary

• Fixes Linux Python bindings can load foreign libHalide.so. #8866: the Python bindings could load a foreign libHalide.so/Halide.dll that shadows the one bundled in the wheel. halide_ depends on it by soname/module name alone, and the dynamic linker's search order (RUNPATH < LD_LIBRARY_PATH on Linux; PATH/default dirs on Windows) lets a same-named library elsewhere on the system take precedence over the bundled copy. Confirmed via readelf that halide_.so has RUNPATH (not RPATH), and reproduced the failure end-to-end in WSL: a garbage libHalide.so on LD_LIBRARY_PATH broke import halide before this fix, and no longer does after it.
• The fix explicitly loads our own copy via ctypes with an absolute path before importing halide_. Both Linux's and Windows's loaders recognize a module already mapped into the process by its soname/module name and reuse it instead of searching again, so halide_'s implicit dependency resolves to our copy regardless of what else is on the search path.
• Separately, cibuildwheel 4.0 made delvewheel repair the Windows default, and it was erroring out trying to locate Halide.dll itself (it doesn't search inside the not-yet-repaired wheel), breaking the Windows pip packaging workflow. Re-enabled with --ignore-existing, which tells delvewheel to treat DLLs already bundled in the wheel as satisfying their own dependencies rather than erroring out. Verified against the current published win_amd64 wheel: with this flag, delvewheel leaves Halide.dll alone but still correctly vendors msvcp140.dll, a genuine external MSVC runtime dependency that isn't currently bundled at all -- so this isn't just a no-op safety net, it closes a real gap in the Windows wheel's self-containedness.

Test plan

• Reproduced Linux Python bindings can load foreign libHalide.so. #8866 in WSL against the published halide==21.0.0 manylinux wheel (garbage libHalide.so on LD_LIBRARY_PATH breaks import)
• Verified the ctypes-preload fix flips that repro from fail to pass, and that basic Halide functionality (Func/realize) still works
• Verified delvewheel repair --ignore-existing against the published win_amd64 wheel: no error, Halide.dll untouched, msvcp140.dll correctly vendored
• CI: Build PyPI package workflow green on all platforms

🤖 Generated with Claude Code

[alexreinking]

No CI actually touches these files pre-merge. I'll merge upon approval.

[codecov]

Codecov Report

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Additional details and impacted files

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