Take the heads' build and packaging from PlatformHeads

[SimonCropp]

BuildMonitor's native build was a copy of DiffEngine's, and fixes made to that copy did not reach this one (#47 and #48 were three of them). PlatformHeads is the one copy, and this takes its 0.1.0.

What changes

• build-native.yml calls PlatformHeads' reusable workflow in place of the jobs it had.
• native/CMakeLists.txt fetches its CMake module, which pins raylib and Dear ImGui, switches off the raylib 6.0 options and links the library. The formats, flags and settings are the ones this file had.
• BuildMonitor.csproj lists a PlatformHead for each RID, and the package's targets publish and bundle them in place of PublishHeads, CopyHeadsToOutput and AddHeadsToPackage. The native head sets PlatformHeadsNativeRuntimes in place of its own runtimes items. BM0001 and BM0002 become PH0001 and PH0002.
• native/build-linux.sh goes: the workflow carries the script, and PlatformHeads keeps it for running by hand.

Three pins name the one tag and move together: the workflow's uses:, the GIT_TAG in native/CMakeLists.txt, and PlatformHeads in Directory.Packages.props.

Checked

Nothing that ships should change, and locally nothing did:

• Built in the manylinux container through the module at 0.1.0, the linux-x64 library is byte for byte the one committed.
• A Release build has no warnings, and its package carries all six heads, each with its own native library and no other.
• Packaging.Tests passes against the snapshot it already had, and BuildMonitor.Tests passes.

This branch's push is the first run of the reusable workflow's macOS and propose jobs. The names of the Build native checks change, since the jobs are now inside a called workflow.