Skip shared Firecracker standby compression

[sjmiller609]

Summary

• skip Firecracker standby snapshot compression when FIEMAP reports shared memory extents
• keep compression enabled for non-shared files and non-Firecracker targets
• add a low-cardinality reason label to snapshot compression job metrics so shared-extent skips are visible without changing disk utilization accounting
• extend the Firecracker warm fork-chain integration test to inspect retained Firecracker snapshot bases for shared extents while preserving the existing fork-to-running and fork-again flow

Tests

• go test ./lib/instances -run 'TestStartCompressionJobSkipsFirecrackerStandbySharedMemory|TestStartCompressionJobDoesNotSkipNonFirecrackerSharedMemory|TestSnapshotCompressionMetrics_RecordAndObserve|TestStartCompressionJobDelayedCancellationRecordsSkipped'
• go test ./lib/forkvm ./lib/hypervisor/firecracker