feat(workspaces): reserve the append canvas for the whole drag — no scrollbar change mid-drag

Previously the grid kept `width: max-content` during a drag, so the canvas
grew/shrunk as the preview placed the tile at each hovered cell — the
scrollbar resized mid-drag, making placement harder. Now the moment a drag
starts the grid's growth axis becomes a FIXED reservation: the append limit
(committed content edge + 3, span-aware) computed at drag start, so the 3
spaces "pop in" immediately, the scroller size is constant for the entire
gesture, and the unused reservation disappears when the drag ends (the
canvas returns to max-content / auto). The dragged tile's span room is
included so its body always stays inside the reserved canvas.

Also: the scroll-measure effect now tracks `dragging`, so canPan and the
scroll indicator re-measure when the reservation appears and disappears.

Test: the drag test now asserts the grid's style is a fixed px width while
dragging (never max-content) and back to max-content after the drop. (This
caught a missing `px` unit that would have silently voided the reservation.)
Update the handoff bullet to describe the reservation.

Verification: svelte-check 0 errors/0 warnings; vitest 1693/1693; biome
clean on touched files; build ok.

1 files changed, 7 insertions, 4 deletions

diff --git a/backend-handoff.md b/backend-handoff.md
index a90a351..e37c7d0 100644
--- a/backend-handoff.md
+++ b/backend-handoff.md
@@ -286,10 +286,13 @@ changes, NO contract changes — the wire types and every endpoint consumed are
   elements to the GRID CONTAINER, which `setPointerCapture`s the pointer and never gets
   re-parented/`pointer-events: none`/unmounted — so move/up stream to it over empty cells and off
   the canvas, and capture can never be lost to a DOM move again.
-- **Drag append bound** (`dragAppendLimit` in `logic/tile-layout.ts` + `cellAt` in
-  `HomeTileGrid.svelte`): the drag target is clamped to at most 3 columns (wide) / rows (tall)
-  past the current content edge (span-aware, measured on the COMMITTED layout so it can't ratchet
-  mid-drag) — the canvas no longer grows endlessly when dragging far right / down.
+- **Drag append bound + reserved canvas** (`dragAppendLimit` in `logic/tile-layout.ts`;
+  `cellAt` + `gridStyle` in `HomeTileGrid.svelte`): the drag target is clamped to at most 3
+  columns (wide) / rows (tall) past the current content edge (span-aware, measured on the
+  COMMITTED layout so it can't ratchet mid-drag). While dragging, the grid's box is a FIXED
+  reservation — the 3 spaces (plus the dragged tile's span room) pop in at drag start and stay
+  constant, so the scroller never changes size mid-drag; the unused spaces vanish when the drag
+  ends.
 - Verification: `typecheck` 0 errors/0 warnings, vitest 1689/1689 (incl. updated
   `tile-layout.test.ts` order assertions — coordinates unchanged), biome clean on touched files,
   `build` succeeds. Browser-only (pointer capture) — needs a human at the page to confirm the