feat(ui): add panelResize geometry for edge and corner resizing

Pure box arithmetic for resizing a floating panel from any of the eight
compass handles: the named edges follow the pointer, the opposite edge
stays pinned, and minimum size plus the viewport margin win. Owned by the
same boundary groups as PanelPositionControls, which wires it next.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/ui/panelResize.test.mjs

@@ -0,0 +1,64 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { resizeBox } from './panelResize.js';
+
+const box = { left: 100, top: 100, width: 400, height: 300 };
+const limits = {
+  minWidth: 200,
+  minHeight: 150,
+  viewportWidth: 1000,
+  viewportHeight: 800,
+};
+
+test('east and south edges grow away from the anchored corner', () => {
+  assert.deepEqual(resizeBox(box, 'se', 50, 20, limits), {
+    left: 100,
+    top: 100,
+    width: 450,
+    height: 320,
+  });
+  assert.deepEqual(resizeBox(box, 'e', -30, 999, limits), {
+    left: 100,
+    top: 100,
+    width: 370,
+    height: 300,
+  });
+});
+
+test('west and north edges keep the opposite edge pinned', () => {
+  assert.deepEqual(resizeBox(box, 'w', -40, 0, limits), {
+    left: 60,
+    top: 100,
+    width: 440,
+    height: 300,
+  });
+  assert.deepEqual(resizeBox(box, 'nw', 40, 30, limits), {
+    left: 140,
+    top: 130,
+    width: 360,
+    height: 270,
+  });
+  assert.deepEqual(
+    resizeBox(box, 'n', -50, 0, limits).top,
+    100,
+    'horizontal motion does not move a north edge',
+  );
+});
+
+test('minimum size stops the moving edge, not the anchored one', () => {
+  const small = resizeBox(box, 'w', 350, 0, limits);
+  assert.deepEqual(small, { left: 300, top: 100, width: 200, height: 300 });
+  assert.equal(resizeBox(box, 'ne', 0, 400, limits).height, 150);
+  assert.equal(resizeBox(box, 'ne', 0, 400, limits).top, 250);
+});
+
+test('the viewport margin caps growth in every direction', () => {
+  assert.equal(resizeBox(box, 'e', 2000, 0, limits).width, 1000 - 6 - 100);
+  assert.equal(resizeBox(box, 's', 0, 2000, limits).height, 800 - 6 - 100);
+  const west = resizeBox(box, 'w', -2000, 0, limits);
+  assert.equal(west.left, 6);
+  assert.equal(west.left + west.width, 500);
+  const north = resizeBox(box, 'n', 0, -2000, limits);
+  assert.equal(north.top, 6);
+  assert.equal(north.top + north.height, 400);
+});