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>

public/grid-layout.ts

@@ -16,15 +16,23 @@
  */
 
 /** A screen layout for the terminal area. Ordered by pane capacity. */
-export type GridLayout = 'single' | 'split-2' | 'grid-4'
+export type GridLayout = 'single' | 'split-2' | 'row-3' | 'grid-4' | 'grid-6'
 
-/** All layouts, in the order the toggle presents them. */
-export const GRID_LAYOUTS: readonly GridLayout[] = ['single', 'split-2', 'grid-4']
+/** All layouts, in the order the toggle presents them (by capacity). */
+export const GRID_LAYOUTS: readonly GridLayout[] = [
+  'single',
+  'split-2',
+  'row-3',
+  'grid-4',
+  'grid-6',
+]
 
 const CAPACITY: Readonly<Record<GridLayout, number>> = {
   single: 1,
   'split-2': 2,
+  'row-3': 3,
   'grid-4': 4,
+  'grid-6': 6,
 }
 
 /** How many panes a layout shows at once. */
@@ -57,6 +65,17 @@ export function isVisibleIndex(
   return idx >= 0 && idx < Math.min(Math.max(0, tabCount), layoutCapacity(layout))
 }
 
+/**
+ * Match the split-grid focus-cycle keybinding (Ctrl+` forward, Ctrl+Shift+`
+ * reverse). Returns the cycle direction, or null if the event isn't the binding.
+ * Extracted from the DOM listener so it's unit-testable in isolation.
+ */
+export function matchFocusCycleKey(e: KeyboardEvent): 1 | -1 | null {
+  if (!e.ctrlKey || e.altKey || e.metaKey) return null
+  if (e.key !== '`' && e.code !== 'Backquote') return null
+  return e.shiftKey ? -1 : 1
+}
+
 /** Minimum viewport width (px) at which multi-pane layouts are offered. Four
  *  terminals need real width; below this the board falls back to 'single'. */
 export const GRID_MIN_WIDTH = 1024
@@ -73,7 +92,9 @@ export function gridAllowed(): boolean {
 const GRID_LAYOUT_KEY = 'web-terminal:grid-layout'
 
 function isGridLayout(v: unknown): v is GridLayout {
-  return v === 'single' || v === 'split-2' || v === 'grid-4'
+  return (
+    v === 'single' || v === 'split-2' || v === 'row-3' || v === 'grid-4' || v === 'grid-6'
+  )
 }
 
 /** Load the persisted layout. Forced to 'single' when the screen is too narrow
@@ -114,7 +135,9 @@ export interface GridToggle {
 const LAYOUT_META: Readonly<Record<GridLayout, { title: string; cells: number }>> = {
   single: { title: 'Single pane', cells: 1 },
   'split-2': { title: 'Side by side — compare two', cells: 2 },
+  'row-3': { title: '1×3 row — three across', cells: 3 },
   'grid-4': { title: '2×2 watch board', cells: 4 },
+  'grid-6': { title: '2×3 grid — six sessions', cells: 6 },
 }
 
 /** A small CSS-drawn layout glyph (N cells), reliable across platforms. */