Final filesystem observation reconstructed root admission from the initial
snapshot's root entries, so an initially absent root (no entry) whose ancestor
became a regular file failed capture with ENOTDIR after the child completed.
A returned initial snapshot now admits every selected root for the run. One
tagged path lookup classifies ENOENT and ENOTDIR as observed absence and other
unavailable lookups (EACCES, ELOOP, ...) as entry_unavailable omissions that
keep the root incomplete. Selected roots that are obstructed or denied before
launch still fail capture.
Fixes#6213
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>