fix(process)!: retain partial filesystem snapshot observations

Keep expected readlink and follow-up lstat failures as typed entry omissions,
including the observed path and errno. Retain directory metadata and sibling
observations, distinguish known identity replacements, and keep cancellation
and unexpected I/O failures terminal. Reuse admitted root metadata and test
cancellation by descriptor state rather than a count of named lookups.

Adapt the symlink race handling proposed in #4587 at fae4e25539.

Closes #4575

BREAKING CHANGE: Filesystem enumeration omission reasons now also include
entry_unavailable and identity_changed. Clients validating the old literal
reason must accept the expanded capture schema.

Co-authored-by: rudrakshagarwal4200-ai <rudrakshagarwal4200@gmail.com>

docs/process-capture.md

@@ -241,6 +241,15 @@ follow also remain unknown. Partial enumeration of one root does not erase
 known effects in another, and unavailable content hashes do not by themselves
 make path absence unknown.
 
+When a follow-up read fails after an entry's metadata was observed, the
+snapshot keeps unaffected observations and records an `entry_unavailable`
+enumeration omission with the root-aliased path and system error code.
+Successfully observed identity replacements use `identity_changed` with a null
+system code. Unstable symlink targets are omitted; already observed directory
+metadata remains available. The affected root stays incomplete, so omitted
+entries do not prove deletion. A path missing at its initial lookup remains an
+observed absence. Cancellation and unexpected I/O failures still stop capture.
+
 Terminal frame `data` is the comparison text after the selected normalization.
 New captures also preserve `raw_data` when normalization changes a PTY chunk;
 otherwise `data` is already the original text. Older captures may lack the