fix(scanner): repair acronym and apostrophe casing on genre tags — #2468

Operator decision: keep the ID3v1 table canonical, fix the casing.

The operator's library carries "Edm", "Idm", "Aor", "Uk Garage", "Uk
Hardcore", "Trap Edm", "Glitch Hop Edm" and "Children'S Music" — an external
tag editor title-cased the whole genre field. The "'S" is the giveaway.

Fixed at SCAN time, not in the display layer: taste_profile.sql reads
tracks.genre directly, so a cosmetic-only fix would leave the taste
vocabulary holding "Edm" while the UI showed "EDM", and any correctly
tagged file would contribute a second, separate tag.

trueUpCasing only ever changes case, never letters, so it cannot silently
turn one genre into a different one — that is what separates it from the
label-remapping idea this task rejected. Two narrow rules:

- A short, evidence-led acronym list, matched case-insensitively so "edm",
  "Edm" and "EDM" all land on "EDM". This is a deliberate exception to the
  project's rule that genre case is exposed as the file says it: "Rock" and
  "rock" still stay separate rows, because folding those is a judgement about
  labels, whereas there is no genre named "Edm".
- Apostrophe suffixes from a FIXED contraction list, so "Children'S" is
  repaired while "O'Brien" and "D'Angelo" keep their capital. A blanket
  "lowercase after an apostrophe" would have broken both.

Matching uses the word's letter core rather than the raw word, so "(Edm)"
and "Edm," are repaired and their punctuation re-attached. Interior
punctuation stays in the core, so "Lo-Fi" and "R&B" are compared whole and
cannot match a fragment by accident. My first version missed this and a test
expecting "(Live EDM)" caught it.

Names resolved from the ID3v1 table are deliberately NOT re-cased, per the
operator's call — entry 40's "AlternRock" stays as the table spells it, with
a test pinning that so a later tidy-up doesn't quietly "fix" it.

tagReadVersion 1 -> 2, so this reaches the existing library on the next scan
rather than new files only. That re-read reuses stored durations, so it costs
tag reads and no ffprobe.

internal/library/genre_test.go

@@ -419,3 +419,102 @@ func equalStrings(a, b []string) bool {
 	}
 	return true
 }
+
+func TestTrueUpCasing(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		// The damage actually present in the operator's library (#2468).
+		{"edm acronym", "Edm", "EDM"},
+		{"idm acronym", "Idm", "IDM"},
+		{"aor acronym", "Aor", "AOR"},
+		{"uk prefix", "Uk Garage", "UK Garage"},
+		{"uk hardcore", "Uk Hardcore", "UK Hardcore"},
+		{"acronym mid-phrase", "Trap Edm", "Trap EDM"},
+		{"acronym at the end", "Glitch Hop Edm", "Glitch Hop EDM"},
+		{"possessive", "Children'S Music", "Children's Music"},
+
+		// Already correct input must be left exactly alone.
+		{"correct acronym", "EDM", "EDM"},
+		{"correct possessive", "Children's Music", "Children's Music"},
+
+		// Case-insensitive, so a lower-cased tag also lands on the canonical
+		// form rather than becoming a third variant.
+		{"lowercase acronym", "edm", "EDM"},
+
+		// Names with an apostrophe followed by a real word are NOT contractions
+		// and must keep their capital — this is why the suffix list is fixed
+		// rather than "lowercase anything after an apostrophe".
+		{"irish surname", "O'Brien Core", "O'Brien Core"},
+		{"french elision", "D'Angelo Soul", "D'Angelo Soul"},
+
+		// Ordinary genres pass through untouched. Case is otherwise exposed as
+		// the file says it — "Rock" vs "rock" stays a real distinction.
+		{"plain", "Alternative Rock", "Alternative Rock"},
+		{"lowercase plain", "rock", "rock"},
+		{"hyphenated", "Lo-Fi Hip Hop", "Lo-Fi Hip Hop"},
+		{"ampersand", "R&B", "R&B"},
+		{"empty", "", ""},
+
+		// Punctuation around a word must not hide the acronym inside it.
+		{"parenthesised acronym", "Hip Hop (Edm)", "Hip Hop (EDM)"},
+		{"acronym with comma", "Edm, Trap", "EDM, Trap"},
+
+		// Interior punctuation stays in the core, so these are compared whole
+		// and cannot match a fragment by accident.
+		{"hyphenated stays whole", "Lo-Fi", "Lo-Fi"},
+		{"ampersand stays whole", "Drum & Bass", "Drum & Bass"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := trueUpCasing(tc.in); got != tc.want {
+				t.Errorf("trueUpCasing(%q) = %q, want %q", tc.in, got, tc.want)
+			}
+		})
+	}
+}
+
+// The casing repair must never touch a name resolved from the ID3v1 table. The
+// operator chose to keep that table canonical, so entry 40's 1990s spelling
+// "AlternRock" stays as-is even though it reads like damage.
+func TestNormaliseGenreValue_CanonicalTableNamesNotRecased(t *testing.T) {
+	if got := normaliseGenreValue("40"); !equalStrings(got, []string{"AlternRock"}) {
+		t.Errorf("bare 40 = %q, want [AlternRock]", got)
+	}
+	if got := normaliseGenreValue("(40)"); !equalStrings(got, []string{"AlternRock"}) {
+		t.Errorf("(40) = %q, want [AlternRock]", got)
+	}
+}
+
+// Casing runs on values that came from the file, including the parenthesised
+// and refinement paths.
+func TestNormaliseGenreValue_CasingAppliedToFileText(t *testing.T) {
+	tests := []struct {
+		in   string
+		want []string
+	}{
+		{"Edm", []string{"EDM"}},
+		{"(17)Uk Garage", []string{"Rock", "UK Garage"}},
+		// Punctuation around the acronym must not hide it — the letter core is
+		// what gets matched, and the trimmed edges are re-attached.
+		{"(Live Edm)", []string{"(Live EDM)"}},
+		{"((Children'S Music", []string{"(Children's Music"}},
+	}
+	for _, tc := range tests {
+		t.Run(tc.in, func(t *testing.T) {
+			if got := normaliseGenreValue(tc.in); !equalStrings(got, tc.want) {
+				t.Errorf("normaliseGenreValue(%q) = %q, want %q", tc.in, got, tc.want)
+			}
+		})
+	}
+}
+
+// Two spellings of one acronym in the same file collapse to a single tag rather
+// than surviving as near-duplicates.
+func TestNormaliseGenres_AcronymVariantsDedupe(t *testing.T) {
+	if got := normaliseGenres([]string{"Edm", "EDM", "edm"}); !equalStrings(got, []string{"EDM"}) {
+		t.Errorf("genres = %q, want [EDM]", got)
+	}
+}