Drop Artist/Album/Title depth assumptions from layout checks

Matching has always been by relative path, so any folder layout works as
long as both libraries use the same one. Remove the depth and loose-root-file
warnings, which flagged valid layouts, and reword the UI and README to say
"same folder layout on both sides" with Artist/Album/Title.ext only as an
example. The "elsewhere" fallback is now described as same top-level folder,
different subfolder. Add a test covering flat and deeply nested layouts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/models.go

@@ -220,7 +220,7 @@ const (
 	KindPathCase  = "path_case" // relative path differs only by case
 	KindExtension = "extension" // same directory and title, different extension
 	KindFuzzy     = "fuzzy"     // same directory, normalized title matches
-	KindElsewhere = "elsewhere" // same artist, different album folder
+	KindElsewhere = "elsewhere" // same top-level folder and title, different subfolder
 	KindNone      = "none"
 )