Edited, moved or deleted images keep their old description: Scan only ever adds to KnownPaths and never prunes stale paths

[matt-edmondson]

What's wrong

Scan's step 4 (ImageDescriber/Verbs/Scan.cs:77-99) only ever adds a path to an existing record's KnownPaths. Nothing in the project removes one: the only writers of KnownPaths are Scan.cs:89, Scan.cs:216 (new record) and Import.MergeKnownPaths, and none of them removes entries. The only File.Exists check is in Import.

Failure scenario

1. Scan describes photo.jpg, whose content hashes to H1. Record H1 gets KnownPaths = [photo.jpg].
2. The user crops or edits photo.jpg. Its content now hashes to H2.
3. Scan again. H2 gets a new record with [photo.jpg], but H1 still lists photo.jpg.
4. Search for words from the old description returns photo.jpg with a description of content it no longer has.
5. Stats over-counts "Total known paths" and "Images found at multiple paths", and Export writes the same path under two hashes.

Files that are deleted or renamed also stay in KnownPaths forever, so Search keeps returning paths that no longer exist.

Suggested fix

After hashing, reconcile each record against the scan:

• For every known path under the scanned root, drop it if the scan did not find it, or if it now hashes to a different value.
• Remove records left with no paths, or keep them and report them in the scan summary (e.g. "N descriptions no longer match any file").

Only paths under the scanned root should be touched, so scanning one folder never prunes paths from another.

Acceptance criteria

• Scan a file, change its bytes, and scan again: the path appears under exactly one hash, the new one.
• Scan a file, delete it, and scan the same root again: the path is no longer listed.
• Paths outside the scanned root are untouched.
• Tests cover all three cases.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. Search returns stale descriptions for edited files and returns paths for deleted ones, and Stats/Export double-count. No data is lost, but the results are wrong in the tool's core use.
• Area / suggested assignment: the Scan verb (reconciling KnownPaths)
• Related: Scan silently skips images in dot-folders and dot-named files (and Hidden/System files on Windows) since the unreadable-folder fix #175 (also in Scan). Pruning must only touch paths under the scanned root, and it must use the same file set Scan silently skips images in dot-folders and dot-named files (and Hidden/System files on Windows) since the unreadable-folder fix #175 fixes. If hidden paths stay skipped, a prune would drop them as "not found". Land Scan silently skips images in dot-folders and dot-named files (and Hidden/System files on Windows) since the unreadable-folder fix #175 first.
• Duplicates: none found.
• In progress: no. The open PR fix: return non-zero exit code when commands fail #173 is about exit codes.

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