feat(workspaces): pinned tiles with displacement reflow + FLIP drag preview

Tiles can now be dropped onto specific cells; empty cells are real
estate that persists (notes/pinned-tiles-plan.md).

Data (v2): each tile carries an optional pinned anchor in a UNIVERSAL
4-row frame (tall folds its 2 columns onto frame column 1; a wide col>1
pin reads as unpinned in tall). v1 documents migrate in memory to
all-unpinned (render-identical) and write back as v2 on the next change.
sanitizeLayout drops garbage anchors.

Packing: pinned-first (first-wins on overlap, one-ply), then unpinned
tiles first-fit into the REMAINING cells — holes between pins count and
never compact. displacePlace: the user-specified semantics — the dragged
tile leaves a gap at its origin (nothing slides backward), anchors at
the drop cell, and the contiguous filled run pushes FORWARD until the
first empty cell absorbs it (X_X + Y@1 → YXX; _XZ_ + Y@2 → _YXZ).
placeAtCell commits the displacement and pins every tile; pinAtFirstEmpty
pins the create-flow tile at the first truly-empty slot;
sortByRecentActivity now also clears pins (an explicit arrangement
reset — the menu item says so).

Drag UX: pointer → cell hit-math (empty cells are drop targets); a
faint dot marks every cell center during a drag (CSS radial-gradient
overlay, one extra pitch of end padding for the append slot); the
dragged tile element itself follows the pointer (lifted via transform,
pointer-events none); a dashed tone-colored ghost marks the landing
footprint. The preview recomputes from the COMMITTED layout on every
hover-cell change (as if the user moved straight from the original to
there) while GSAP Flip animates from wherever the tiles currently are
(re-entrant — mid-flight positions captured). Escape/pointercancel
discards the preview and flies the tile home; a drop settles it onto
its new cell.

Tests: displacement (both user examples + internal moves + spans + tall
stream), pinned pack (anchors, gaps kept, first-wins, universal-frame
mapping, deleted-leaves-holes), v1→v2 migration + anchor sanitization,
store placeAtCell/no-op write guard, Escape-cancels-drag component test.
1689 green.

1 files changed, 11 insertions, 8 deletions

diff --git a/src/features/workspaces/logic/drag-controller.ts b/src/features/workspaces/logic/drag-controller.ts
index 03dcade..a1877aa 100644
--- a/src/features/workspaces/logic/drag-controller.ts
+++ b/src/features/workspaces/logic/drag-controller.ts
@@ -12,9 +12,11 @@
  *   so the two gestures never fight. Release while lifted = DROP.
  *
  * Pure: `(phase, event, options) → { phase, emit }`. The component owns the
- * reactive `$state` and performs the emits (navigate / select / reorder).
+ * reactive `$state` and performs the emits (navigate / select / place).
  */
 
+import type { Cell } from "./tile-layout";
+
 /** Where the interaction currently stands. */
 export type DragPhase =
   | { readonly kind: "idle" }
@@ -32,7 +34,8 @@ export type DragPhase =
       readonly id: string;
       readonly x: number;
       readonly y: number;
-      readonly targetIndex: number;
+      /** The grid CELL under the pointer (null = not over the canvas yet). */
+      readonly targetCell: Cell | null;
       readonly touch: boolean;
     };
 
@@ -40,7 +43,7 @@ export type DragPhase =
 export type DragEmit =
   | { readonly kind: "tap"; readonly id: string }
   | { readonly kind: "lift"; readonly id: string }
-  | { readonly kind: "drop"; readonly id: string; readonly targetIndex: number };
+  | { readonly kind: "drop"; readonly id: string; readonly targetCell: Cell | null };
 
 /** Events the component dispatches (DOM events + the long-press timer firing). */
 export type DragEvent =
@@ -55,7 +58,7 @@ export type DragEvent =
       readonly type: "pointer-move";
       readonly x: number;
       readonly y: number;
-      readonly targetIndex: number;
+      readonly targetCell: Cell | null;
     }
   | { readonly type: "pointer-up" }
   | { readonly type: "long-press" }
@@ -105,7 +108,7 @@ export function nextDragPhase(
             id: phase.id,
             x: event.x,
             y: event.y,
-            targetIndex: event.targetIndex,
+            targetCell: event.targetCell,
             touch: phase.touch,
           },
         };
@@ -116,7 +119,7 @@ export function nextDragPhase(
             ...phase,
             x: event.x,
             y: event.y,
-            targetIndex: event.targetIndex,
+            targetCell: event.targetCell,
           },
         };
       }
@@ -130,7 +133,7 @@ export function nextDragPhase(
       if (phase.kind === "dragging") {
         return {
           phase: IDLE,
-          emit: { kind: "drop", id: phase.id, targetIndex: phase.targetIndex },
+          emit: { kind: "drop", id: phase.id, targetCell: phase.targetCell },
         };
       }
       return { phase };
@@ -145,7 +148,7 @@ export function nextDragPhase(
             id: phase.id,
             x: phase.x,
             y: phase.y,
-            targetIndex: -1,
+            targetCell: null,
             touch: true,
           },
           emit: { kind: "lift", id: phase.id },