player bar: track changes morph shape into shape, not up from the dots

The arrival morph honestly started from whatever the lane showed — and on
a track change the lane always showed dots, because the waveform empties
the moment the queue moves. Every switch therefore played the same
grow-from-zero, when the picked motion was one waveform transforming into
another.

A track change now HOLDS the outgoing shape for ARRIVE_HOLD_MS (600ms),
painted in unplayed grey since the new track owns the transport. A quick
arrival — a local file, a cached stream — lands inside the hold and the
morph runs directly from the old bars into the new ones; a slow stream
load falls to the dots-and-swell story when the hold expires, exactly as
before. The rail mirrors the hold with the same shared constant, and
suppresses the loading swell while holding so the keyframes cannot fight
the held heights.

Measured across a cached track switch: the lane's mean height never left
16.9-17.4 (the dots sit at 2, the old dip hit 5.2) while the per-column
fingerprint changed — the shape transformed without ever falling.

src/lib/waveshape.js

@@ -54,6 +54,12 @@ export const RISE_CADENCE_MS = 40; // gap between one bar starting and the next
  * the previous shape — straight into the new one, all at once, one motion.
  * The streaming rise keeps its left-to-right story; this is only coverage 1. */
 export const ARRIVE_MS = 1000;
+/** How long the OUTGOING waveform holds after a track change. A quick
+ * arrival — a local file, a cached stream — lands inside this window and
+ * morphs shape→shape, one waveform transforming into the other; a slow load
+ * falls to the dots-and-swell story when the hold expires. Without the hold,
+ * the cleared lane always morphed up from the dots. */
+export const ARRIVE_HOLD_MS = 600;
 /** Resting/queued bars render the muted grey at this opacity — the rail's
  * pending dim, shared so the two lanes' dots match. */
 export const PENDING_ALPHA = 0.45;