feat(tiles): draggable column and row dividers

The grid now places every tile explicitly (grid-column / grid-row, reading
order) with a 6px divider track between columns and between rows, instead of
relying on DOM order and a gap. Track sizes are fractions (grid-template fr
values) that reset to equal whenever the column or row count changes.

Dragging a divider trades size between the two tracks either side, each kept
at the minimum tile size (the pure dragTrackFractions in constants.js, always
computed from the fractions the drag started with, so it cannot drift). The
affected tiles reflow locally at most once per animation frame, with no PTY
resize; each hears exactly one fit (one PTY resize) at pointer-up, and tiles
in other tracks hear nothing. Pointer capture keeps the drag on the divider,
the body locks the resize cursor and text selection for its duration, and
closing the grid or removing a tile mid-drag tears it down, as the split's
divider does. A zoomed grid shows no dividers.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/mocks/tile-grid-vm.ts

@@ -61,6 +61,11 @@ export class FakeEl {
   type = '';
   focus = vi.fn();
   select = vi.fn();
+  setPointerCapture = vi.fn();
+  releasePointerCapture = vi.fn();
+  removeEventListener(type: string, fn: (ev: unknown) => void) {
+    this.listeners[type] = (this.listeners[type] ?? []).filter((f) => f !== fn);
+  }
   appendChild(child: FakeEl) {
     child.remove();
     child.parentElement = this;
@@ -95,7 +100,6 @@ export class FakeEl {
   addEventListener(type: string, fn: (ev: unknown) => void) {
     (this.listeners[type] ||= []).push(fn);
   }
-  removeEventListener() {}
   dispatch(type: string, ev: unknown = {}) {
     for (const fn of this.listeners[type] ?? []) fn(ev);
   }
@@ -169,6 +173,8 @@ export function advanceClock(ms: number) {
 export const clockNow = () => clock;
 /** Every callback the code under test handed a PerformanceObserver, newest last. */
 export const perfObserverCallbacks: Array<(list: { getEntries(): unknown[] }) => void> = [];
+/** Animation-frame callbacks the code under test queued (id = index + 1); a test runs them. */
+export const rafCallbacks: Array<() => void> = [];
 /** What the code under test deferred with requestIdleCallback; a test runs them. */
 export const idleCallbacks: Array<() => void> = [];
 export const windowStub: Record<string, unknown> = {
@@ -194,7 +200,10 @@ const context = vm.createContext({
   clearInterval: vi.fn(),
   setTimeout: (fn: () => void, ms?: number) => globalThis.setTimeout(fn, ms),
   clearTimeout: (id: ReturnType<typeof setTimeout>) => globalThis.clearTimeout(id),
-  requestAnimationFrame: vi.fn(),
+  requestAnimationFrame: (cb: () => void) => rafCallbacks.push(cb),
+  cancelAnimationFrame: (id: number) => {
+    if (id > 0) rafCallbacks[id - 1] = () => {};
+  },
   requestIdleCallback: (cb: () => void) => idleCallbacks.push(cb),
   HTMLCanvasElement: class HTMLCanvasElement {},
   WebSocket: { OPEN: 1 },
@@ -295,6 +304,7 @@ export function makeGridApp(ids: string[] = ['s-a', 's-b', 's-c']): GridApp {
 export function resetGridHarness() {
   FakeTile.all = [];
   idleCallbacks.length = 0;
+  rafCallbacks.length = 0;
   localStore.clear();
   windowStub.innerWidth = 2400;
   section.children = [];