feat(grid): split-grid v2 — 1×3/2×3, drag-to-quadrant, Ctrl+` cycle, maximize

Builds on the v1 watch board:
- Two more layouts: row-3 (1×3) and grid-6 (2×3). A single `grid` marker class on
  #term now carries the shared cell chrome so it no longer enumerates each lay-*.
- Ctrl+` cycles the focused quadrant (Ctrl+Shift+` reverses); matchFocusCycleKey
  is an exported pure helper so it's unit-tested. No-op / passthrough in single mode.
- Per-quadrant ⛶ maximize: the focused cell expands to fill the grid as an absolute
  overlay while siblings stay live behind it; follows focus, resets on layout
  change / tab close.
- Drag a tab from the tab bar onto a quadrant to assign it there (reuses the
  existing tab-drag dragIndex); grid-only.

Adversarial review (3 lenses → per-finding verify, incl. a headless-Chrome repro)
caught a HIGH: maximizing via `grid-column/row: 1/-1` shoved siblings into implicit
rows — a strip instead of fullscreen AND a spurious resize to backgrounded live
PTYs. Fixed with an absolute-overlay (`position:absolute; inset:0`), then verified
in real Chromium (Playwright): maximized cell fills #term, siblings 0px size delta.
Also: silence a covered pane's pending pulse under maximize; coarse-pointer target
for .cell-max. Verified: typecheck + build:web clean, 1579 tests pass.

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

test/grid-layout.test.ts

@@ -20,6 +20,7 @@ import {
   loadGridLayout,
   saveGridLayout,
   mountGridToggle,
+  matchFocusCycleKey,
 } from '../public/grid-layout.js'
 
 const KEY = 'web-terminal:grid-layout'
@@ -62,11 +63,30 @@ describe('grid-layout: pure logic', () => {
   it('layoutCapacity maps each layout to its pane count', () => {
     expect(layoutCapacity('single')).toBe(1)
     expect(layoutCapacity('split-2')).toBe(2)
+    expect(layoutCapacity('row-3')).toBe(3)
     expect(layoutCapacity('grid-4')).toBe(4)
+    expect(layoutCapacity('grid-6')).toBe(6)
   })
 
-  it('GRID_LAYOUTS is ordered single → split-2 → grid-4', () => {
-    expect(GRID_LAYOUTS).toEqual(['single', 'split-2', 'grid-4'])
+  it('GRID_LAYOUTS is ordered by capacity', () => {
+    expect(GRID_LAYOUTS).toEqual(['single', 'split-2', 'row-3', 'grid-4', 'grid-6'])
+  })
+
+  it('visibleIndices honors the higher-capacity layouts', () => {
+    expect(visibleIndices(9, 'row-3')).toEqual([0, 1, 2])
+    expect(visibleIndices(9, 'grid-6')).toEqual([0, 1, 2, 3, 4, 5])
+    expect(visibleIndices(4, 'grid-6')).toEqual([0, 1, 2, 3])
+  })
+
+  it('matchFocusCycleKey recognizes Ctrl+` (fwd) and Ctrl+Shift+` (rev), else null', () => {
+    const k = (init: KeyboardEventInit): KeyboardEvent => new KeyboardEvent('keydown', init)
+    expect(matchFocusCycleKey(k({ ctrlKey: true, key: '`' }))).toBe(1)
+    expect(matchFocusCycleKey(k({ ctrlKey: true, shiftKey: true, key: '`' }))).toBe(-1)
+    expect(matchFocusCycleKey(k({ ctrlKey: true, code: 'Backquote' }))).toBe(1)
+    expect(matchFocusCycleKey(k({ key: '`' }))).toBeNull() // no ctrl
+    expect(matchFocusCycleKey(k({ ctrlKey: true, metaKey: true, key: '`' }))).toBeNull() // meta excluded
+    expect(matchFocusCycleKey(k({ ctrlKey: true, altKey: true, key: '`' }))).toBeNull() // alt excluded
+    expect(matchFocusCycleKey(k({ ctrlKey: true, key: 'a' }))).toBeNull() // wrong key
   })
 
   it('visibleIndices returns the first min(tabCount, capacity) indices', () => {