player bar: the track-change morph stops being a coin flip

Two independent fragilities each collapsed the shape→shape morph back to
a grow-from-dots, which is why it only sometimes worked.

The hold was 600ms against an arrival whose timing is DECODE time: the
same track costs ~5ms once the session holds its waveform in memory but
hundreds of ms to over a second on the first cold decode. 2000ms covers
the cold case with room; fresh-stream searches take well past that, so
the loading story still gets the lane.

And the hold bailed on a bar-count mismatch — but the lane's width follows
the song TITLE now, so switching between a long and a short name resizes
the lane mid-gap and the guard threw the held shape away. Held and morphed
heights are refitted to the new count instead (nearest neighbour — it is a
moving start point, on screen for under a second).

Four consecutive cached switches measured after the fix: minimum lane
height 15.1 on every one (dots are ~2, the old dip hit ~5). Gapless
playback is untouched — the hold and morph are pure painting, and the
sink-append path never waits on the lane.

src/lib/waveshape.js

@@ -58,8 +58,14 @@ export const ARRIVE_MS = 1000;
  * 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;
+ * the cleared lane always morphed up from the dots.
+ *
+ * 2000, up from 600: an arrival's timing is decode time, and the same track
+ * costs ~5ms once the session has its waveform in memory but hundreds of ms
+ * to over a second on the first, cold decode — a 600ms hold made the morph
+ * a coin flip on exactly the tracks it was built for. Fresh-stream searches
+ * take well past 2s, so the loading story still gets the lane. */
+export const ARRIVE_HOLD_MS = 2000;
 /** 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;