Fix snapshot disk utilization classification for Firecracker

[sjmiller609]

Summary

• classify Firecracker standby snapshots that use memory, memory.zst, or memory.lz4
• keep recognizing the legacy memory-ranges* snapshot names
• update disk utilization and monitoring tests to cover the live Firecracker layout

Testing

• go test ./lib/diskutilization
• go test ./lib/resources -run TestStartMonitoringPublishesDiskUtilizationFromCachedSnapshot

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Note

Medium Risk
Changes snapshot classification logic that feeds disk utilization metrics, which could affect monitoring/alerting and capacity dashboards if misclassified. Scope is small and covered by updated unit/integration-style tests.

Overview
Updates snapshot disk utilization classification to recognize Firecracker standby snapshot layouts by treating memory, memory.zst, and memory.lz4 as valid snapshot memory artifacts (alongside legacy memory-ranges*).

Refreshes documentation and expands tests to cover the new Firecracker filenames, including monitoring’s cached disk-utilization metric emission.

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