Add actual disk utilization metric by component

[sjmiller609]

Summary

• add hypeman_disk_utilization_bytes for actual filesystem usage by storage component
• keep hypeman_resources_disk_breakdown_bytes as the existing allocation/provisioned view
• add tests for sparse-file accounting, snapshot classification, and cached refresh behavior

Testing

• go test ./lib/diskutilization ./lib/resources
• validated the new collector on dev-yul-hypeman-1 against the live filesystem without deploying Hypeman changes there first
• live server usage matched essentially exactly:
  • images: 658.6 GiB
  • oci_cache: 108.3 GiB
  • volumes: 1.9 GiB
  • rootfs_overlays: 89.3 GiB
  • volume_overlays: 0 GiB
  • snapshot_uncompressed: 288.2 GiB
  • snapshot_compressed: 86.4 GiB
  • snapshot_other: 0 GiB
• all buckets matched exactly except rootfs_overlays, which differed by only 208 KiB across separate runs, consistent with normal live filesystem drift
• runtime on that host was about 100-120 ms per collection, with scrapes still remaining cheap because the metric is measured on the refresh loop and served from cached in-memory values

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Note

Medium Risk
Introduces a new filesystem-walking collector in the resource monitoring refresh loop; bugs or performance regressions could affect monitoring refresh latency and metric correctness, but it does not change core scheduling or data paths.

Overview
Adds a new lib/diskutilization collector that measures actual filesystem allocated bytes (sparse-aware via stat.Blocks) for Hypeman storage components, including snapshot classification (compressed/uncompressed/other).

Integrates this into resource monitoring by caching the collected breakdown in the monitoring snapshot and exporting it as a new Prometheus/OTel gauge hypeman_disk_utilization_bytes (keeping the existing hypeman_resources_disk_breakdown_bytes as the provisioned/allocation view), with tests covering sparse-file accounting, snapshot classification, and that scrapes read cached values until the next refresh.

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