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/style.css

@@ -327,8 +327,9 @@ body {
 }
 
 /* ── Split-grid layouts (desktop watch board) ────────────────────────── */
-#term.lay-split-2,
-#term.lay-grid-4 {
+/* Shared cell chrome keys off the single `grid` marker so it is independent of
+ * how many concrete layouts exist; each `lay-*` only sets the grid template. */
+#term.grid {
   display: grid;
   gap: 6px;
   padding: 6px;
@@ -337,20 +338,40 @@ body {
   grid-template-columns: 1fr 1fr;
   grid-template-rows: 1fr;
 }
+#term.lay-row-3 {
+  grid-template-columns: 1fr 1fr 1fr;
+  grid-template-rows: 1fr;
+}
 #term.lay-grid-4 {
   grid-template-columns: 1fr 1fr;
   grid-template-rows: 1fr 1fr;
 }
-#term.lay-split-2 .term-cell,
-#term.lay-grid-4 .term-cell {
+#term.lay-grid-6 {
+  grid-template-columns: 1fr 1fr 1fr;
+  grid-template-rows: 1fr 1fr;
+}
+#term.grid .term-cell {
   position: relative;
   inset: auto;
   border: 1px solid var(--border);
   border-radius: var(--radius);
   background: var(--bg);
 }
-#term.lay-split-2 .cell-head,
-#term.lay-grid-4 .cell-head {
+/* Maximized quadrant fills the whole grid as an ABSOLUTE overlay (taken out of
+ * grid flow so the siblings keep their placement — spanning grid tracks instead
+ * would shove auto-placed siblings into implicit rows, resizing their live PTYs).
+ * #term is a positioned containing block, so inset:0 covers the whole area. */
+#term.grid .term-cell.maximized {
+  position: absolute;
+  inset: 0;
+  z-index: 4;
+}
+/* A tab dragged from the tab bar is hovering this quadrant. */
+#term.grid .term-cell.drag-target {
+  outline: 2px dashed var(--accent);
+  outline-offset: -3px;
+}
+#term.grid .cell-head {
   display: flex;
   align-items: center;
   gap: 8px;
@@ -361,6 +382,20 @@ body {
   cursor: pointer;
   user-select: none;
 }
+.cell-max {
+  border: 0;
+  background: transparent;
+  color: var(--text-faint);
+  cursor: pointer;
+  font-size: 13px;
+  line-height: 1;
+  padding: 2px 4px;
+  border-radius: 5px;
+}
+.cell-max:hover {
+  color: var(--text);
+  background: var(--surface-3);
+}
 .cell-name {
   font-weight: 600;
   color: var(--text);
@@ -388,16 +423,14 @@ body {
 }
 
 /* Focus ring — the quadrant that owns keyboard / keybar / voice / approvals. */
-#term.lay-split-2 .term-cell.focused,
-#term.lay-grid-4 .term-cell.focused {
+#term.grid .term-cell.focused {
   border-color: var(--accent);
   box-shadow:
     0 0 0 1px var(--accent),
     var(--shadow);
 }
 /* Pending glow — a non-focused quadrant is waiting for approval. */
-#term.lay-split-2 .term-cell.pending:not(.focused),
-#term.lay-grid-4 .term-cell.pending:not(.focused) {
+#term.grid .term-cell.pending:not(.focused) {
   border-color: var(--amber);
   animation: cell-pending 2.4s ease-in-out infinite;
 }
@@ -413,15 +446,13 @@ body {
   }
 }
 @media (prefers-reduced-motion: reduce) {
-  #term.lay-split-2 .term-cell.pending,
-  #term.lay-grid-4 .term-cell.pending {
+  #term.grid .term-cell.pending {
     animation: none;
   }
 }
 
 /* Inline per-quadrant approve footer. */
-#term.lay-split-2 .cell-approve,
-#term.lay-grid-4 .cell-approve {
+#term.grid .cell-approve {
   display: flex;
   align-items: center;
   gap: 8px;
@@ -548,10 +579,17 @@ body {
 .grid-glyph.gl-split-2 {
   grid-template-columns: 1fr 1fr;
 }
+.grid-glyph.gl-row-3 {
+  grid-template-columns: 1fr 1fr 1fr;
+}
 .grid-glyph.gl-grid-4 {
   grid-template-columns: 1fr 1fr;
   grid-template-rows: 1fr 1fr;
 }
+.grid-glyph.gl-grid-6 {
+  grid-template-columns: 1fr 1fr 1fr;
+  grid-template-rows: 1fr 1fr;
+}
 .grid-glyph i {
   background: currentColor;
   border-radius: 1px;
@@ -566,6 +604,10 @@ body {
     width: 36px;
     height: 34px;
   }
+  .cell-max {
+    padding: 6px 9px;
+    font-size: 15px;
+  }
 }
 
 /* ── Key bar (rounded chips) ─────────────────────────────────────── */