Function patch_key 

pub fn patch_key(
    req: &PatchRequest<'_>,
) -> Result<PatchOutcome, ConfigWriteError>

What: Apply a single key/value patch to a Pacsea config file.

Inputs:

• req: PatchRequest describing target file, key (with aliases), value, and dry-run flag.

Output:

• PatchOutcome::NoChange when the file already holds the requested value.
• PatchOutcome::Written when the file was updated atomically.
• PatchOutcome::DryRun when req.dry_run is true and a change would have been made.

Errors

• ConfigWriteError::Invalid when req.key is empty after trimming.
• ConfigWriteError::Io when reading the existing file or performing the atomic temp-file write/rename fails.

Details:

• Comments and unrelated keys are preserved: only the matched line is rewritten in place. When no existing line matches, the canonical key = value pair is appended.
• Aliases are migrated in the same pass: an alias line on disk is rewritten with the primary key.
• File is bootstrapped from the matching skeleton when missing or empty.
• Writes go through [atomic_write] (temp file + rename, mode 0o600 on Unix), so a partially-written file is never visible.