containerd2.dmverity.test

dmverity: add structured dmverity_format debug logs

Triage instrumentation on top of the commitBlock-format fix
(93cd2c3e6). Every event uses tag=dmverity_format so it can be filtered
with 'journalctl -u containerd | grep dmverity_format'.

Events emitted:
- FormatLayerBlob: enter, idempotent_hit, success (with elapsedMs),
  rollback (when format fails after pre-allocate)
- formatDmverityLayer (differ delegate): differ_invoke
- snapshotter.Commit: commit_format_path (taking commitBlock branch),
  commit_differ_path (debug-level, layer.erofs already present),
  commitblock_done (after commitBlock returns), commit_skip_off,
  commit_format_failed_auto, commit_format_failed_on,
  commit_unknown_mode
- commitBlock: commitblock_enter
- applyDmverityPolicy: policy_reject (with blob size + mtime +
  metadata path so we can distinguish 'format never ran' from
  'format ran but sidecar wrong shape')

All log lines are Info level except policy_reject (Error), rollback
and commit_format_failed_auto (Warn).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>