The Swift bridge previously called MTLLogState, MTLResidencySet,
MTL4*, MTLSamplerDescriptor.reductionMode and related macOS 15+/26+
APIs unconditionally, breaking compilation on macos-14 and macos-15
GitHub Actions runners. Add #available(macOS 15, *) /
#available(macOS 26, *) guards on every affected @_cdecl thunk so
the bridge compiles against the macOS 14 SDK with reduced
functionality and against the macOS 15 SDK with the residency-set /
log-state surface, and only requires macOS 26 SDK for MTL4* /
sampler-reduction-mode features.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>