Add macOS ARM64 (Apple Silicon) support

[abedegno]

Summary

• Add pre-built libadlmidi.dylib for osx-arm64 runtime
• Add OSPlatform.OSX handling in DllImportFixture.cs and TestApp/Program.cs

Test plan

• Builds on macOS (Apple M4 Pro, .NET 10)
• Native library loads successfully via P/Invoke
• XMI music files convert and play in Godot 4.6.1 mono project
• Verify no regression on Windows/Linux

[csinkers]

Looks good, would you be able to add some info on the macOS build process to the readme.md? I'd like to add a github workflow at some point that will grab the libADLMIDI library at a particular commit and build it + commit so we have a more auditable / verifiable native build system, so some macOS steps would definitely help me write that in the absence of any suitable hardware.

[abedegno]

@csinkers

Pushed a new commit adding a macOS build section to the README. Verified end-to-end by building libADLMIDI HEAD (commit 73d046d) with CMake 4.2.1 / Apple clang 17 on macOS arm64 - produces a working libADLMIDI.dylib with the same install name and dependencies as the shipped binary.

Two things worth flagging for the CI workflow you're planning:

1. WITH_MUS_SUPPORT is no longer a recognised cmake variable in current libADLMIDI - it just emits a "manually-specified variable not used" warning. I dropped it from the new macOS section; you may want to drop it from the Linux section too.
2. Three new emulators (ESFMU, MAME, YMFM) default to ON in current libADLMIDI. The macOS section I added explicitly disables them to match the "Nuked only" intent, but the existing Linux/Windows commands don't - so those binaries are silently shipping with three extra emulators that aren't exposed by the .NET Emulator enum. Probably worth adding -DUSE_ESFMU_EMULATOR=OFF -DUSE_MAME_EMULATOR=OFF -DUSE_YMFM_EMULATOR=OFF to the Linux command (and the Windows checklist) when you write the CI workflow, to keep all three platforms consistent.

[csinkers]

Perfect, thanks!