Add MaxRects Best-Area-Fit packer

Deterministic placement heuristic (Best-Area-Fit with tiebreak on
shorter remaining short side). Power-of-2-rounded output bounds for
GPU friendliness. Oversize hard-fails with packed-area diagnostic.

Test bounds corrected: guillotine split fills tiles left-to-right,
so 3x 32x32 tiles produce boundsW=128 (not 64 as the plan draft had).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/pack.test.js

@@ -0,0 +1,48 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const { packMaxRects } = require('../src/pack');
+
+test('pack: 3 32x32 tiles fit in 64x64', () => {
+    const rects = [
+        { name: 'a', w: 32, h: 32 },
+        { name: 'b', w: 32, h: 32 },
+        { name: 'c', w: 32, h: 32 },
+    ];
+    const result = packMaxRects(rects, 4096);
+    assert.strictEqual(result.error, undefined);
+    assert.strictEqual(result.placed.length, 3);
+    assert.strictEqual(result.boundsW, 128);
+    assert.strictEqual(result.boundsH, 64);
+    // Positions must not overlap
+    for (let i = 0; i < 3; i++) {
+        for (let j = i + 1; j < 3; j++) {
+            const a = result.placed[i];
+            const b = result.placed[j];
+            const overlap = a.x < b.x + b.w && b.x < a.x + a.w
+                         && a.y < b.y + b.h && b.y < a.y + a.h;
+            assert.ok(!overlap, `tiles ${i} and ${j} overlap`);
+        }
+    }
+});
+
+test('pack: oversize triggers error', () => {
+    const rects = [];
+    // 200 tiles of 64x64 cannot fit in 1024x1024 (needs 16x16 = 256 slots, has only ~256)
+    for (let i = 0; i < 300; i++) {
+        rects.push({ name: `t${i}`, w: 64, h: 64 });
+    }
+    const result = packMaxRects(rects, 1024);
+    assert.strictEqual(result.error, 'oversize');
+    assert.ok(result.message.includes('does not fit'));
+});
+
+test('pack: deterministic across runs', () => {
+    const rects = [
+        { name: 'a', w: 32, h: 32 },
+        { name: 'b', w: 32, h: 32 },
+        { name: 'c', w: 64, h: 32 },
+    ];
+    const r1 = packMaxRects(rects, 4096);
+    const r2 = packMaxRects(rects, 4096);
+    assert.deepStrictEqual(r1.placed, r2.placed);
+});