Rename Target::OS's OSX to MacOS, deprecate the old name

[abadams]

Summary

• Halide's Target::OS used the increasingly out-of-date string/enum name osx for macOS. This renames the canonical form to MacOS/"macos" throughout the codebase.
• Target::OSX is kept as a [[deprecated]] alias enumerator equal to Target::MacOS, so old C++ code still compiles but gets a -Wdeprecated-declarations warning.
• Target string parsing still accepts "osx" as an alias for "macos", but emits a user_warning when it does, and Target::to_string() now always prints "macos".
• Updated the Python binding (hl.TargetOS.OSX) similarly: kept as a deprecated alias, following the same pattern used for TailStrategy::Predicate.
• Updated CMake helpers (HalideTargetHelpers.cmake, HalideGeneratorHelpers.cmake), the autoscheduler/packaging CMakeLists that referenced -osx triples, and docs, so host-detected macOS targets now come out as -macos end-to-end.
• Swept all internal uses of Target::OSX/hl.TargetOS.OSX (tests, tutorials, codegen) over to the new MacOS name so nothing in-tree triggers the new deprecation warning.

In the future, using the deprecated OSX/"osx" spelling could be made a hard error, but for now it's just deprecated.

Test plan

• g++ -fsyntax-only sanity checks on all touched .cpp/.h files (no LLVM-linked build available in this environment)
• Verified Target::OSX now triggers -Wdeprecated-declarations
• Verified std::map iteration order makes to_string() print "macos" (not "osx") for the parsed/host target
• ctest -R correctness_target (please run in CI / on a full build)

[alexreinking]

LGTM with one possible enhancement

[mcourteaux]

I actually wouldn't mind seeing Target:macOS and Target::iOS, to be honest. But perhaps clang-tidy is going to not like proper capitalization...

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.06%. Comparing base (4c5525c) to head (3771224).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/CPlusPlusMangle.cpp	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9451      +/-   ##
==========================================
- Coverage   70.12%   70.06%   -0.06%     
==========================================
  Files         261      261              
  Lines       79818    79937     +119     
  Branches    19445    19479      +34     
==========================================
+ Hits        55972    56009      +37     
- Misses      17991    18077      +86     
+ Partials     5855     5851       -4     

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