Run CI on pull requests, on the vendored branch, and on Windows

[dougchansan]

Three gaps in the current workflow, each of which let a break through.

Nothing ran on a pull request — only push to main and manual dispatch. So a change could reach a branch other people vendor before anything compiled it. The missing include that stopped core building (#7) got in exactly that way.

Only main was watched on push, but moderngekko-vendor is the branch ModernGekko actually vendors. The branch being consumed was the one not being checked.

Windows was not built at all. That is how the precompiled-header flags being applied to C sources — GXRuntime handed a C++ PCH, fatal error C1853 — reached the vendored branch and stopped the downstream launcher from compiling.

Also: the unit tests exist and nothing ran them. tests is EXCLUDE_FROM_ALL, so it has to be asked for by name. Linux and Windows now build it and run ctest.

Concurrency cancels superseded runs, since these builds are not cheap.

Verified

YAML parses; triggers, watched branches and jobs are what they should be. The tests target name and its EXCLUDE_FROM_ALL status are read from Source/UnitTests/CMakeLists.txt rather than assumed.

I have not been able to rehearse the runners themselves. The macOS and Linux jobs are unchanged apart from the test steps, so they should behave as before; the Windows job is new and may need a fixup on its first run.

Note

The Windows job will fail until #7 lands — that is the point of it. Merge #7 first.

How this fits

These land together as a set: savestates working end to end, the Windows build and test suite being usable at all, and CI so none of it regresses unnoticed. This PR is RC #10.

MG #18	Bump vendored RecompCore	prerequisite - without it the launcher cannot compile on Windows (C sources get a C++ PCH)
RC #7	Interpreter.cpp include	prerequisite - core does not compile without it
RC #8	In-game File/View menu and hotkeys	introduces Core/SavestateLayout.h, the one definition of where savestates live, what they are called and how they are ordered
MG #21	Launcher savestate picker, --load-state	consumes that header, so the launcher and the in-game menu cannot disagree
MG #22	Make the test suite pass on Windows	land before CI, or the first run is red on day one
MG #23	Build and test on PRs, three platforms	the reason the rest stayed broken unnoticed
RC #10	CI on PRs, the vendored branch, and Windows	same gap, other repo; its Windows job fails until RC #7 lands
MG #19	--opt-level, default -O2	independent
MG #20	Cache-domain affinity	independent

Suggested order: #18 and RC #7, then RC #8, then MG #21; MG #22 before MG #23. The rest are independent.

Verified on three platforms: Windows (MSVC + clang), Linux (g++ 15.2, x86_64) and macOS 26.1 (clang, arm64). The portable pieces - the savestate layout and its tests, frontend_config, dol_patch - build and pass on all three. Windows-only pieces are guarded and their tests registered behind if(WIN32).