fix(highway_3d): dolly back so the fret-number row can't clip off the bottom (#633)

The heat-coloured fret-number row is drawn as a band BELOW the board
(sY(lowest) - S_GAP*1.4), but camUpdate's self-correcting framing only
anchors the board CENTRE to the lower third of the screen and reserves no
headroom for that row. So a tight zoom on a centred active span (worst
mid-neck; fine at either end of the neck) pushes the numbers past the
bottom edge -- which is why testers saw it "only when centered" and "not
every song." Tilt can't fix it (it would only trade a bottom clip for a
top clip); the vertical-extent problem at tight zoom needs camera distance.

Add a fret-row fit guard: project the row band with the final camera and,
when it falls below FRET_ROW_FIT_NDC_MIN, raise a capped, hysteretic
_fretRowFitBoost applied to the curDist lerp target (the span-driven
tgtDist still owns zooming IN). The boost rises promptly (proportional to
the deficit), relaxes lazily past a deadband, and is capped at
FRET_ROW_FIT_BOOST_MAX (+60%) so the zoom can't pop or hunt. It cooperates
with the tilt loop (pull-back shrinks the scene, tilt keeps the centre
anchored) and yields entirely to the Camera Director free-cam. Surgical:
passages where the row is already visible never trigger it, so framing is
unchanged everywhere it already worked.

plugin.json 3.30.0 -> 3.30.2 (screen.js cache-buster; 3.30.1 is taken by the
FPS-counter PR). Tests: tests/js/highway_3d_camera_framing.test.js
(guard constants, the boosted curDist lerp, the projected-row hysteresis,
free-cam yield).

Fixes #632


Claude-Session: https://claude.ai/code/session_01QbexxfTt8q2tAn436MqGWF

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

plugins/highway_3d/plugin.json

@@ -1,7 +1,7 @@
 {
     "id": "highway_3d",
     "name": "3D Highway",
-    "version": "3.30.1",
+    "version": "3.30.2",
     "type": "visualization",
     "bundled": true,
     "script": "screen.js",