Refuse to remove Omarchy while its EFI partition is mounted

[joshuaswarren]

Removal now refuses an EFI partition that is already mounted, before it deletes anything.

What happened

On a 14-inch M2 Max (apple,j414c, macOS 27.0, installer 2.0.10) the Omarchy EFI partition was still mounted through FSKit's msdos module from a backup taken earlier. Remove Omarchy read it in place. Then diskutil eraseVolume free none disk0s4 forced it off. The unified log shows the unmount request with option 0x00080000 (force), then from com.apple.fskit.msdos:

flush_meta returned -536870208
-[FATVolume unmountWithReplyHandler:]_block_invoke: Failed to meta flush, error Error Domain=NSPOSIXErrorDomain Code=5
-[FATVolume unmountWithReplyHandler:]_block_invoke: Failed to clean dirty bit, error Error Domain=NSPOSIXErrorDomain Code=5

The partitions were removed, but from then on mount, diskutil list and diskutil unmount hung. The app looked frozen. Disk commands answered again only after fskit_agent and its msdos extension were stopped by hand, about 16 minutes later.

main already reads an unmounted EFI partition from its raw device (a2f898b). The mounted case still reads in place and then erases a mounted partition.

Change

• withEFIPartition refuses a mounted EFI partition. The message names the mount point and says to restart the Mac, because macOS doesn't mount the EFI partition at startup. An unmounted one is read from the raw device as before.
• Execution reads the evidence again before it deletes anything, so a partition mounted after review is refused too.
• The executor comment said diskutil refuses a busy volume. The log shows it force-unmounts, so the comment now says that.

Tests

testMountedEFIPartitionIsRefusedBeforeAnythingIsErased covers both review and execution. On main it fails: review accepts the mounted partition, and execution goes on to apfs deleteContainer. With this change it passes. testOlderInstallIsPlannedEndToEndFromDiskutilAndFilesReadInPlace is now ...WithTheStubReadInPlace and mounts only the stub, because a mounted EFI partition is no longer read in place.

Run on an M1 Ultra Mac Studio (Mac13,2), macOS 26.6.2, Xcode 27.0, Swift 6.4:

• xcrun swift-format lint --strict --recursive Package.swift Sources Tests: clean
• swift test: 147 + 3 + 520 + 8 tests, 0 failures (1 existing skip)
• swift test -c release: 136 + 3 + 520 + 5 tests, 0 failures
• ./test/all on Linux: passed

I didn't reproduce the hang on hardware again. The evidence is the log from the M2 Max.

[joshuaswarren]

Rebased onto the current main and the checks pass. Ready when you are.