Write a proper build for HelloWasm app

[alexreinking]

The single-file CMakeLists.txt worked but hand-invoked emcc directly, manually reassembling include paths/flags instead of using CMake's own Emscripten toolchain integration, and never actually produced a working demo end-to-end (index.html was copied into the build dir, but main.js/sw.js, which that copied index.html references, never were, so opening it from the build dir 404s).

Replace it with a proper CMake super-build: generator/ builds the Halide generator with the host compiler; app/ is configured with Emscripten's own CMake toolchain file and builds core.cpp as an ordinary add_executable, linked against the AOT pipeline libraries via target_link_libraries -- no more hand-rolled emcc command line. index.html/main.js/sw.js get staged into the build dir alongside the compiled output, fixing the demo for real.

Converting to a super-build means it can no longer be included directly from apps/CMakeLists.txt (same as HelloiOS), so it gets a dedicated build+test step in testing-macos.yml instead of riding along with the aggregate apps/ ctest run.

Also moves reaction_diffusion_generator.cpp into generator/ and core.cpp into app/, matching every other multi-piece app in this repo.

Fixes #8731

Breaking changes

None.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Commits include AI attribution where applicable (see Code of Conduct)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.27%. Comparing base (f932856) to head (ecbf1b4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9285      +/-   ##
==========================================
- Coverage   70.32%   70.27%   -0.06%     
==========================================
  Files         258      258              
  Lines       79160    79160              
  Branches    18997    18997              
==========================================
- Hits        55672    55631      -41     
- Misses      17802    17806       +4     
- Partials     5686     5723      +37     

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