build(dev): generate a 50k-track library and measure a running app

Plan 007 phase 4 is verified by measurement, not by assertion, and
there was no way to produce a number: the fixture library is a few
dozen tracks and cannot show any of the findings.

- `cmd/gentestdata -bulk N` (`make bulkdata`) writes a ~50 000-track
  library in 11 s / 466 MB by encoding six clips once and copying
  them, while still tagging every file through `backend/tagwriter` —
  a library the app cannot read back measures nothing.
- `make sandbox-seed-bulk` seeds from it through the same script and
  the same discipline as any other seed: by running the app and
  waiting for the real scan.
- `e2e/perf/measure.mjs` (`make perf LABEL=x`, `make perf-compare`)
  takes fourteen measurements against a running app and writes them
  to a gitignored `.dev/perf/<label>.json`. It wraps every bound Go
  method, so "did that refetch the library" is a fact rather than an
  inference, and records `longtask` entries, which is where a 25 MB
  JSON parse on the main thread shows up and nowhere else.

It is not a spec and does not run in CI.

cmd/gentestdata/audio.go

@@ -161,7 +161,19 @@ func requireFFmpeg() error {
 // the library's cover-art deduplication is supposed to collapse into a
 // single stored blob.
 func coverJPEG(key string) ([]byte, error) {
-	img := image.NewRGBA(image.Rect(0, 0, coverSizePx, coverSizePx))
+	return coverJPEGSized(key, coverSizePx)
+}
+
+// coverJPEGSized is coverJPEG at an explicit edge length.  The bulk
+// library uses a larger one, because a 64 px cover cannot show the
+// difference between rendering the original artwork and rendering the
+// thumbnail tier that exists for the purpose.
+// coverJPEGSized is coverJPEG at an explicit edge length.  The bulk
+// library uses a larger one, because a 64 px cover cannot show the
+// difference between rendering the original artwork and rendering the
+// thumbnail tier that exists for the purpose.
+func coverJPEGSized(key string, px int) ([]byte, error) {
+	img := image.NewRGBA(image.Rect(0, 0, px, px))
 
 	// A per-key hue derived from the key's bytes, plus a diagonal
 	// band, so covers are distinguishable by eye in a screenshot.
@@ -177,8 +189,8 @@ func coverJPEG(key string) ([]byte, error) {
 		A: 255,
 	}
 
-	for y := range coverSizePx {
-		for x := range coverSizePx {
+	for y := range px {
+		for x := range px {
 			c := base
 			if (x+y)%16 < 8 {
 				c.R /= 2