perf(highway_3d): forceSinglePass on transparent DoubleSide quads

The retrace after the label-swap fix showed getParameters unchanged
(~2.5s / ~4% throttled main thread) — the real driver is Three r158+'s
transparent-DoubleSide two-pass path: renderBufferDirect renders such
objects back side then front side, setting material.needsUpdate BOTH
times, i.e. a full getParameters/program-cache lookup twice per object
per frame, plus double draw calls. (Found by reading the two-pass
branch in the vendored three.module.min.js right next to the
getParameters call site.)

All 18 transparent DoubleSide materials in this renderer are flat
unlit quads — technique markers, sustain rails, chord frames, lane
planes, halo bars — where the two-pass self-occlusion ordering buys
nothing. Declare forceSinglePass: true on all of them.

Also corrects the _setLabelMap comment's churn attribution (that fix
removes the label-swap contribution; this one removes the dominant
source). Plugin 3.31.1 -> 3.31.2.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugins/highway_3d/plugin.json

@@ -1,7 +1,7 @@
 {
     "id": "highway_3d",
     "name": "3D Highway",
-    "version": "3.31.1",
+    "version": "3.31.2",
     "type": "visualization",
     "bundled": true,
     "script": "screen.js",