feat(library): album loudness from the tracks' summed block histograms (M464 #4996)

Album-mode normalization plays a whole album at one gain. That gain comes
from album_loudness (migration 0066): BS.1770's gated loudness over every
block on the album, computed by summing the tracks' stored histograms and
gating the sum. No audio is decoded again. Album true peak is the loudest
track's.

- Recomputed by the loudness worker each tick, after the track pass and
  whether or not analysis is switched on. ListAlbumsNeedingLoudness lists
  albums whose md5 over (present track id, measurement version and time) no
  longer matches the stored digest. One comparison covers every way
  membership changes (scan retag, duplicate merge, delete, missing and
  restored) without hooking each.
- No album value until every present track has a settled measurement, so
  an album's gain doesn't shift mid-listen as the rest is measured. Silent
  and unreadable tracks count as settled.
- Rows for albums with no present track left are dropped.
- The parser and the merge share trimBins.

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

internal/library/loudness.go

@@ -220,7 +220,6 @@ var (
 // histogram, and the closing summary.
 type ebur128Parser struct {
 	bins      [loudnessHistBins]int32
-	blocks    int
 	inSummary bool
 	summary   map[string]string
 	lastLines []string
@@ -273,7 +272,6 @@ func (p *ebur128Parser) line(raw string) {
 	}
 	bin := int(math.Round((v - loudnessHistFloor) * loudnessHistPerLU))
 	p.bins[min(bin, loudnessHistBins-1)]++
-	p.blocks++
 }
 
 func (p *ebur128Parser) tail() string {
@@ -281,18 +279,24 @@ func (p *ebur128Parser) tail() string {
 }
 
 func (p *ebur128Parser) histogram() blockHistogram {
-	if p.blocks == 0 {
-		return blockHistogram{}
-	}
+	return trimBins(&p.bins)
+}
+
+// trimBins turns a full-range bin array into a histogram trimmed to its
+// occupied bins; an empty array gives an empty histogram.
+func trimBins(bins *[loudnessHistBins]int32) blockHistogram {
 	first, last := 0, loudnessHistBins-1
-	for p.bins[first] == 0 {
+	for first <= last && bins[first] == 0 {
 		first++
 	}
-	for p.bins[last] == 0 {
+	if first > last {
+		return blockHistogram{}
+	}
+	for bins[last] == 0 {
 		last--
 	}
 	counts := make([]int32, last-first+1)
-	copy(counts, p.bins[first:last+1])
+	copy(counts, bins[first:last+1])
 	return blockHistogram{start: int16(first), counts: counts}
 }