feat(workspaces): bound the drag append area — 3 columns past the content edge

Dragging a tile far right grew the wide canvas endlessly (the target cell
resolved from the pointer with no upper bound), making precise placement
impossible. Add a pure `dragAppendLimit(placements, geometry, extra = 3)`:
the drag target may aim at most 3 columns (wide) / rows (tall) past the
current span-aware content edge. The bound is measured on the COMMITTED
layout — never the live preview — so it can't ratchet mid-drag; it steps
forward only when a drop commits new content. cellAt applies it on the
growth axis (wide: col, tall: row); the fixed cross-axis clamps are
unchanged. Exported via the unit index.

Verification: svelte-check 0 errors/0 warnings; vitest 1693/1693 (4 new
dragAppendLimit tests); biome clean on touched files; build ok.

1 files changed, 31 insertions, 0 deletions

diff --git a/src/features/workspaces/logic/tile-layout.test.ts b/src/features/workspaces/logic/tile-layout.test.ts
index a5d58c0..b538373 100644
--- a/src/features/workspaces/logic/tile-layout.test.ts
+++ b/src/features/workspaces/logic/tile-layout.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from "vitest";
 import {
   clampSpan,
   displacePlace,
+  dragAppendLimit,
   fillOrderKey,
   firstEmptySlot,
   type GridGeometry,
@@ -524,6 +525,36 @@ describe("placeAtCell", () => {
   });
 });
 
+describe("dragAppendLimit", () => {
+  it("caps a wide drag at 3 columns past the right-most content edge (span-aware)", () => {
+    // b spans cols 3-4 → the right-most occupied column is 4; +3 = 7.
+    const placements: Placement[] = [
+      { id: "a", col: 1, row: 1, colSpan: 1, rowSpan: 1 },
+      { id: "b", col: 3, row: 1, colSpan: 2, rowSpan: 1 },
+    ];
+    expect(dragAppendLimit(placements, WIDE)).toBe(7);
+  });
+
+  it("caps a tall drag at 3 rows past the bottom-most content edge (span-aware)", () => {
+    // b spans rows 3-4 → the bottom-most occupied row is 4; +3 = 7.
+    const placements: Placement[] = [
+      { id: "a", col: 1, row: 1, colSpan: 1, rowSpan: 2 },
+      { id: "b", col: 2, row: 3, colSpan: 1, rowSpan: 2 },
+    ];
+    expect(dragAppendLimit(placements, TALL)).toBe(7);
+  });
+
+  it("an empty grid still allows the first tiles within the first 3 columns", () => {
+    expect(dragAppendLimit([], WIDE)).toBe(3);
+    expect(dragAppendLimit([], TALL)).toBe(3);
+  });
+
+  it("honors a custom extra", () => {
+    const placements: Placement[] = [{ id: "a", col: 1, row: 1, colSpan: 1, rowSpan: 1 }];
+    expect(dragAppendLimit(placements, WIDE, 5)).toBe(6);
+  });
+});
+
 describe("sortByRecentActivity", () => {
   it("re-applies the recent-activity order (descending)", () => {
     const tiles = [tile("a", 0), tile("b", 1), tile("c", 2)];