feat(highway): render per-note bend curve (bnv) on 2D + 3D (#532)

PR-B of the bend-shape feature (feedpak §6.2.1). Both highways drew a bend
from the scalar `bn` only; now they trace the authoritative `bnv` curve
([{t, v}]) when present and fall back to the `bn` arc/envelope otherwise.

2D (static/highway.js drawNote): when a note carries `bnv`, draw the real
shape as a contour above the gem (round-trip rises then falls, pre-bend
starts high, release descends — `bt` is implicit in the point shape), with
an arrowhead only when the gesture ends rising. `bnvNormalizedPoints` maps
{t,v} to a 0..1 x span. The scalar-arrow path is preserved unchanged as the
fallback; the peak label is unchanged.

3D (plugins/highway_3d/screen.js): `bnvSampleAt` linearly interpolates the
curve (clamped to its endpoints) and `bendSemisAtTime` samples it when
present, else keeps the synthetic rise→hold→release envelope from `bn`. The
chevron count still comes from the peak. Fixed a stale-scratch hazard: the
reused `_scrChordNote` now resets `bnv`/`bt` (omit-when-default) after
Object.assign, mirroring the existing `fhm` reset, so a chord note without a
curve can't inherit the previous note's contour.

Render-only — no wire/schema change. Pure helpers covered by
tests/js/highway_bend_curve.test.js (interp, clamping, round-trip,
degenerate/empty); node --check passes on both files; full tests/js green.

Part of got-feedback/feedback#334

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/js/highway_bend_curve.test.js

@@ -0,0 +1,77 @@
+// Behavioural tests for the per-note bend-curve (bnv, §6.2.1) render helpers:
+// `bnvNormalizedPoints` (static/highway.js, 2D glyph) and `bnvSampleAt`
+// (plugins/highway_3d/screen.js, 3D Y gesture). Both are pure, so we extract
+// the function source by brace-matching and eval it in isolation.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+function extractFn(src, name) {
+    const start = src.indexOf('function ' + name);
+    assert.ok(start >= 0, `function ${name} must exist`);
+    const open = src.indexOf('{', start);
+    let depth = 0;
+    for (let i = open; i < src.length; i++) {
+        if (src[i] === '{') depth++;
+        else if (src[i] === '}' && --depth === 0) return src.slice(start, i + 1);
+    }
+    throw new Error(`unbalanced braces extracting ${name}`);
+}
+
+function loadFn(file, name) {
+    const src = fs.readFileSync(path.join(__dirname, '..', '..', file), 'utf8');
+    return new Function('"use strict";' + extractFn(src, name) + `\nreturn ${name};`)();
+}
+
+const bnvNormalizedPoints = loadFn('static/highway.js', 'bnvNormalizedPoints');
+const bnvSampleAt = loadFn('plugins/highway_3d/screen.js', 'bnvSampleAt');
+
+// ── bnvNormalizedPoints (2D) ─────────────────────────────────────────────────
+
+test('bnvNormalizedPoints normalizes t to 0..1 across the span', () => {
+    const pts = bnvNormalizedPoints([
+        { t: 0.5, v: 0 }, { t: 1.0, v: 2 }, { t: 1.5, v: 0 }]);
+    assert.deepEqual(pts, [
+        { x: 0, v: 0 }, { x: 0.5, v: 2 }, { x: 1, v: 0 }]);
+});
+
+test('bnvNormalizedPoints handles degenerate/empty input', () => {
+    assert.deepEqual(bnvNormalizedPoints([]), []);
+    assert.deepEqual(bnvNormalizedPoints(null), []);
+    // All-same-t span collapses x to 0 (no divide-by-zero).
+    assert.deepEqual(bnvNormalizedPoints([{ t: 1, v: 1 }, { t: 1, v: 2 }]),
+        [{ x: 0, v: 1 }, { x: 0, v: 2 }]);
+});
+
+// ── bnvSampleAt (3D) ─────────────────────────────────────────────────────────
+
+test('bnvSampleAt linearly interpolates between points', () => {
+    const bnv = [{ t: 0, v: 0 }, { t: 1, v: 2 }];
+    assert.equal(bnvSampleAt(bnv, 0.5), 1);   // midpoint
+    assert.equal(bnvSampleAt(bnv, 0.25), 0.5);
+});
+
+test('bnvSampleAt clamps to the endpoints', () => {
+    const bnv = [{ t: 0.2, v: 1 }, { t: 0.8, v: 3 }];
+    assert.equal(bnvSampleAt(bnv, 0), 1);     // before first
+    assert.equal(bnvSampleAt(bnv, 5), 3);     // after last
+});
+
+test('bnvSampleAt traces a round-trip curve up then back down', () => {
+    const bnv = [{ t: 0, v: 0 }, { t: 0.5, v: 2 }, { t: 1, v: 0 }];
+    assert.equal(bnvSampleAt(bnv, 0.25), 1);  // rising
+    assert.equal(bnvSampleAt(bnv, 0.5), 2);   // peak
+    assert.equal(bnvSampleAt(bnv, 0.75), 1);  // falling
+});
+
+test('bnvSampleAt returns 0 for an empty/invalid curve', () => {
+    assert.equal(bnvSampleAt([], 0.5), 0);
+    assert.equal(bnvSampleAt(null, 0.5), 0);
+});
+
+test('bnvSampleAt tolerates a zero-width segment (duplicate t)', () => {
+    const bnv = [{ t: 0, v: 0 }, { t: 0.5, v: 1 }, { t: 0.5, v: 2 }, { t: 1, v: 2 }];
+    assert.equal(bnvSampleAt(bnv, 0.5), 1);   // first matching segment wins
+});