feat(sim): pure altitude-scrub math module + node unit suite

Per plan Task 1. position->segment, frac->currentTime, audio gains, and
integer-crossing detection; 9 node --test cases green.

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

simulator/unit/README.md

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+# Simulator unit tests
+
+Pure-logic JS unit tests (no browser), run with Node's built-in test runner:
+
+    node --test simulator/unit/
+
+`scrub.test.js` covers `simulator/static/scrub.js` — the pure altitude-scrub math
+(position→segment, frac→currentTime, audio gains, integer-crossing detection).

simulator/unit/scrub.test.js

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+"use strict";
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const S = require("../static/scrub.js");
+
+test("clamp01 bounds to [0,1]", () => {
+  assert.equal(S.clamp01(-0.2), 0);
+  assert.equal(S.clamp01(0.4), 0.4);
+  assert.equal(S.clamp01(1.5), 1);
+});
+
+test("wrapIndex wraps both directions", () => {
+  assert.equal(S.wrapIndex(0, 5), 0);
+  assert.equal(S.wrapIndex(5, 5), 0);
+  assert.equal(S.wrapIndex(-1, 5), 4);
+  assert.equal(S.wrapIndex(7, 5), 2);
+});
+
+test("segmentOf splits floor and fraction", () => {
+  assert.deepEqual(S.segmentOf(2), { lo: 2, frac: 0 });
+  const s = S.segmentOf(2.4);
+  assert.equal(s.lo, 2);
+  assert.ok(Math.abs(s.frac - 0.4) < 1e-9);
+});
+
+test("accumToPos maps knob degrees to position", () => {
+  assert.equal(S.accumToPos(2, 0, 72), 2);
+  assert.equal(S.accumToPos(2, 72, 72), 3);
+  assert.equal(S.accumToPos(2, -36, 72), 1.5);
+});
+
+test("fracToTime clamps and scales by duration", () => {
+  assert.equal(S.fracToTime(0, 1.5), 0);
+  assert.equal(S.fracToTime(0.5, 1.5), 0.75);
+  assert.equal(S.fracToTime(1.2, 1.5), 1.5);
+  assert.equal(S.fracToTime(0.5, 0), 0);
+});
+
+test("crossfadeGains splits energy by fraction", () => {
+  assert.deepEqual(S.crossfadeGains(0), { from: 1, to: 0 });
+  assert.deepEqual(S.crossfadeGains(1), { from: 0, to: 1 });
+  assert.deepEqual(S.crossfadeGains(0.25), { from: 0.75, to: 0.25 });
+});
+
+test("integerCrossings: none while inside a segment", () => {
+  assert.deepEqual(S.integerCrossings(2.1, 2.9), []);
+  assert.deepEqual(S.integerCrossings(2.0, 2.5), []);
+  assert.deepEqual(S.integerCrossings(3.0, 2.5), []);
+});
+
+test("integerCrossings: ascending and descending single crossings", () => {
+  assert.deepEqual(S.integerCrossings(2.4, 3.0), [{ index: 3, dir: 1 }]);
+  assert.deepEqual(S.integerCrossings(2.0, 3.0), [{ index: 3, dir: 1 }]);
+  assert.deepEqual(S.integerCrossings(3.4, 3.0), [{ index: 3, dir: -1 }]);
+});
+
+test("integerCrossings: multiple crossings in travel order", () => {
+  assert.deepEqual(S.integerCrossings(2.4, 4.1), [{ index: 3, dir: 1 }, { index: 4, dir: 1 }]);
+  assert.deepEqual(S.integerCrossings(3.2, 1.8), [{ index: 3, dir: -1 }, { index: 2, dir: -1 }]);
+});