feat: reject RAM disk growth when system memory runs low

[coldhighsun]

Summary

• Backlog item was originally phrased as "GC.AddMemoryPressure", but RAM disk content is plain managed byte[] chunks the GC already tracks, so that API would misrepresent them as unmanaged memory with no real benefit.
• Instead: query real system-wide available physical memory and reject a growing Create/Overwrite/SetFileSize/Write with STATUS_INSUFFICIENT_RESOURCES once fewer than 256 MiB would remain after the allocation — independent of the disk's own configured capacity, since a disk can have capacity headroom while the host machine itself is nearly out of RAM.
• Moved the existing SystemMemoryInfo (Win32 GlobalMemoryStatusEx wrapper) from the App layer into ManagedDrive.Core.Diagnostics so MemoryFileSystem can call it directly; the one existing App call site is unaffected since Core.Diagnostics is already a global using there.
• Added an optional availableMemoryProvider constructor parameter to MemoryFileSystem for injecting a fake available-memory value in tests.
• The 256 MiB reserve is a fixed constant this round, not user-configurable.

Test plan

• dotnet build -c Release — 0 warnings, 0 errors
• dotnet test tests/ManagedDrive.Tests -c Release --filter "FullyQualifiedName~MemoryFileSystemLowMemoryGuardTests" — 7/7 passed
• dotnet test tests/ManagedDrive.Tests -c Release — 374/374 passed
• dotnet format --verify-no-changes --no-restore — clean