fix: keep the mirror stable across Lidarr upgrades and renames

Three defects in how the mirror tracked its source, all of which show up on a
library that something else reorganises.

Pruning probed the source tree for a mirror file's original name, trying each
known extension in turn. A source saved as .FLAC was never found, so its mirror
file was deleted as an orphan and re-encoded on the next pass, for ever.
Pruning now works from the set of paths the pass actually accounted for, which
cannot disagree with the walk over letter case or extension coverage.

Two sources could also claim one mirror path -- 01 Song.flac beside a leftover
01 Song.mp3, which is what an interrupted upgrade leaves behind. Both encoded
to the same destination, whichever finished last won the race, and every later
pass found the other one stale. The best-quality source now wins, ties break on
path, and the loser is logged.

The source mtime was read after encoding rather than before. A file still being
written when the pass reached it would be stamped with its final mtime while
holding truncated audio, and would never be revisited.

Adds regression tests for all three, plus the format-upgrade, album-rename and
whole-library-deletion cases, each verified to fail before the change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -28,6 +28,29 @@ mtime, so a file is stale exactly when the two differ. There is no database to
 fall out of step with the library, which matters when something else — Lidarr,
 in this case — is the thing that owns and reorganises it.
 
+### What that means for Lidarr
+
+| Lidarr does this                        | The mirror does this                                                             |
+| --------------------------------------- | -------------------------------------------------------------------------------- |
+| Replaces a file with a better rip       | Re-encodes in place. Same path in, same path out, so no duplicate                |
+| Upgrades MP3 to FLAC                    | Both map to the same `.mp3` mirror path, so the old one is overwritten           |
+| Renames a track, album or artist folder | Old path pruned, new path encoded. Correct, but it re-encodes rather than moving |
+| Deletes an album or artist              | Every orphaned mirror file is deleted and the emptied directories go too         |
+
+Pruning is driven by what the pass actually found, not by guessing source
+filenames from mirror ones: a `.FLAC` source would not be found by a search for
+`.flac`, and the mirror file would be deleted and rebuilt on alternate passes
+for ever.
+
+If two sources want the same mirror path — a `01 Song.flac` next to a leftover
+`01 Song.mp3`, which is what an interrupted upgrade leaves — the better format
+wins, ties break on path, and the loser is logged. Without that rule both
+encode to the same destination and every pass finds one of them stale.
+
+The mtime is read _before_ encoding rather than after. A file still being
+written when the pass reaches it would otherwise be stamped with its final
+mtime while holding truncated audio, and never be revisited.
+
 Encodes are written to a temporary file and renamed into place, so an
 interrupted run cannot leave a truncated MP3 that the next run mistakes for
 finished work. A lock file in the mirror root stops two passes overlapping.