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.go

@@ -89,7 +89,7 @@ func normaliseGenreValue(v string) []string {
 	for strings.HasPrefix(v, "(") {
 		// "((" is the spec's escape for a literal "(" — the rest is plain text.
 		if strings.HasPrefix(v, "((") {
-			return append(out, strings.TrimSpace(v[1:]))
+			return append(out, trueUpCasing(strings.TrimSpace(v[1:])))
 		}
 		end := strings.IndexByte(v, ')')
 		if end < 0 {
@@ -106,7 +106,7 @@ func normaliseGenreValue(v string) []string {
 			if err != nil {
 				// Parenthesised but not a reference, e.g. "(Live)". Keep the
 				// whole remainder as written.
-				return append(out, v)
+				return append(out, trueUpCasing(v))
 			}
 			if name, ok := id3v1GenreName(n); ok {
 				out = append(out, name)
@@ -120,11 +120,104 @@ func normaliseGenreValue(v string) []string {
 	}
 	if n, err := strconv.Atoi(v); err == nil {
 		if name, ok := id3v1GenreName(n); ok {
+			// Canonical table name, deliberately NOT re-cased — see id3v1Genres.
 			return append(out, name)
 		}
 		return out
 	}
-	return append(out, v)
+	return append(out, trueUpCasing(v))
+}
+
+// genreAcronyms are tokens that belong in caps. Tag editors that title-case the
+// whole genre field turn "EDM" into "Edm" and "UK Garage" into "Uk Garage", and
+// the operator's library carries all of these (#2468).
+//
+// Matched case-INSENSITIVELY, so "edm", "Edm" and "EDM" all land on "EDM". That
+// is a deliberate, narrow 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 would be a judgement about labels. Fixing a token that
+// is unambiguously an initialism is not — there is no genre named "Edm".
+//
+// Kept short and evidence-led. Add a token here only when a real library shows
+// it damaged; a speculative list risks mangling a word that legitimately looks
+// like an acronym.
+var genreAcronyms = map[string]string{
+	"edm": "EDM",
+	"idm": "IDM",
+	"aor": "AOR",
+	"uk":  "UK",
+	"us":  "US",
+	"ebm": "EBM",
+}
+
+// contractionSuffixes are the word-endings that follow an apostrophe in normal
+// English. Title-casing capitalises the letter after ANY non-letter, which is
+// how "Children's Music" became "Children'S Music".
+//
+// Deliberately a fixed list rather than "lowercase whatever follows an
+// apostrophe": that broader rule would break "O'Brien" and "D'Angelo", which are
+// correctly capitalised after the apostrophe.
+var contractionSuffixes = map[string]bool{
+	"s": true, "t": true, "re": true, "ll": true, "ve": true, "d": true, "m": true,
+}
+
+// trueUpCasing repairs casing damage done by tag editors that title-case the
+// genre field. It only ever changes case, never the letters — so it cannot
+// silently turn one genre into a different one, which is what separates it from
+// the label-remapping idea #2468 rejected.
+func trueUpCasing(s string) string {
+	if s == "" {
+		return s
+	}
+	words := strings.Split(s, " ")
+	for i, w := range words {
+		if w == "" {
+			continue
+		}
+		// Match the word's letter core, not the raw word, so surrounding
+		// punctuation doesn't hide the acronym: "(Edm)" and "Edm," both need
+		// fixing, and re-attaching the trimmed edges keeps them intact.
+		lead, core, trail := splitWordCore(w)
+		if fixed, ok := genreAcronyms[strings.ToLower(core)]; ok {
+			words[i] = lead + fixed + trail
+			continue
+		}
+		words[i] = fixApostrophe(w)
+	}
+	return strings.Join(words, " ")
+}
+
+// splitWordCore peels leading and trailing non-alphanumerics off a word.
+// Interior punctuation stays in the core, so "Lo-Fi" and "R&B" are compared
+// whole rather than being split into fragments that might match by accident.
+func splitWordCore(w string) (lead, core, trail string) {
+	isCore := func(r rune) bool {
+		return (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9')
+	}
+	start := 0
+	for start < len(w) && !isCore(rune(w[start])) {
+		start++
+	}
+	end := len(w)
+	for end > start && !isCore(rune(w[end-1])) {
+		end--
+	}
+	return w[:start], w[start:end], w[end:]
+}
+
+// fixApostrophe lowercases a capitalised contraction or possessive suffix:
+// "Children'S" -> "Children's". Leaves "O'Brien" alone, since "Brien" is not a
+// contraction suffix.
+func fixApostrophe(w string) string {
+	idx := strings.LastIndexByte(w, '\'')
+	if idx <= 0 || idx == len(w)-1 {
+		return w
+	}
+	suffix := w[idx+1:]
+	if !contractionSuffixes[strings.ToLower(suffix)] {
+		return w
+	}
+	return w[:idx+1] + strings.ToLower(suffix)
 }
 
 func id3v1GenreName(n int) (string, bool) {