Implement Phase 10a - Prop Scaling

- Added specifications for Phase 10a, introducing permanent size changes to `.prop` documents and reversible uniform instance scaling in Sandbox EDIT mode.
- Updated `PropCanvas`, `PropAnchorData`, `PropEditor`, `StageGizmos`, `SandboxStage`, and `StageSelection` classes to support new scaling features.
- Implemented document transactions for scaling, including normalization to a 96 px maximum dimension.
- Introduced a new **Transform Whole Prop** tool in the Prop Editor for moving, rotating, and resizing props.
- Created unit tests for the new scaling functionality, validating shape scaling, anchor transformations, and whole-prop interactions.
- Ensured compatibility with existing prop formats and maintained the integrity of the document structure during scaling operations.

docs/prop_editor_user_guide.md

@@ -20,9 +20,10 @@ The window is divided into these areas:
 | Area | Purpose |
 | --- | --- |
 | `📁 File` and `👁️ View` | File operations and canvas view reset |
-| `Grid`, `Snap`, and `🎯 Center Prop` | Shared grid, snapping, and prop placement settings |
+| `Grid` and `Snap` | Shared grid spacing and snapping settings |
 | `Prop Name` | The name stored in the `.prop` file |
 | Mode bar | Switch between `🖐️ Arrange`, `✏️ Reshape`, and `📌 Attach` |
+| Arrange controls | Whole-prop transform, normalization, centering, and the stickman guide |
 | Authoring canvas | Draw, select, transform, and reshape the prop |
 | Inspector | Material, shape, primitive, and anchor controls |
 | Holding Preview | A private stickman-and-prop preview shown in Attach mode |
@@ -75,11 +76,6 @@ The Holding Preview has its own `Reset View` button; see [Holding Preview](#hold
 | --- | --- |
 | `Grid` | Sets the visible grid spacing and the snap increment, from 1 to 512. |
 | `Snap` | Enables or disables snapping for shape transforms and anchor movement. |
-| `🎯 Center Prop` | Moves the complete prop so the combined shape bounds are centered on the fixed local origin. It also translates every non-origin anchor by the same amount and focuses the canvas on the origin. |
-
-Center Prop is a deliberate one-time command. The editor does not recenter
-automatically after drawing, reshaping, or moving a shape. Empty and already
-centered props are unchanged.
 
 ## Prop name and modes
 
@@ -96,6 +92,38 @@ Arrange is the mode for selecting, moving, rotating, scaling, mirroring, and
 ordering complete shapes. The Add Shape inspector and the shape Erase button
 are available in this mode.
 
+#### Arrange controls
+
+The Arrange mode top bar keeps the whole-prop actions in this order:
+
+1. `✥ Transform Whole Prop`
+2. `📏 Normalize Size`
+3. `🎯 Center Prop`
+4. `Show Stickman Guide`
+
+The scale textbox and Apply Scale button are not part of the editor. Use
+Normalize Size for the standard document size, or use Transform Whole Prop for
+direct canvas manipulation. Normalize Size fits the shape-point bounds to a
+96 px maximum dimension while preserving the fixed origin and proportional
+anchor offsets. Center Prop separately moves the shape bounds and editable
+anchors around the canonical origin. It is a deliberate one-time command: the
+editor does not recenter automatically after drawing, reshaping, or moving a
+shape, and empty or already centered props are unchanged.
+
+With Transform Whole Prop enabled, drag visible shape geometry to move the
+whole prop, drag the dot below the union bounds to rotate it around the
+canonical origin, or drag a corner cross to resize it uniformly. Snap rounds
+the move delta to the current grid. Hold **Ctrl** while rotating to snap to
+15-degree increments. Grip and other editable anchor markers follow each
+operation. The tool previews from the starting document and commits once when
+the gesture is released; **Esc** cancels the active preview. The canonical
+origin itself does not move.
+
+Transform Whole Prop and Normalize Size are hidden in Reshape and Attach.
+Center Prop and Show Stickman Guide remain available in those modes. Turning
+Transform Whole Prop off restores the selected shape and normal per-shape
+Arrange gizmos.
+
 When a shape is selected, the canvas draws:
 
 - a two-tone selection outline and bounding box;
@@ -113,7 +141,9 @@ the selected shape.
 ### `✏️ Reshape`
 
 Reshape is for editing individual vertices. It does not move the whole shape
-and does not show the Add Shape section or Erase Shape button. When the canvas
+and does not show the Add Shape section, Erase Shape button, Normalize Size,
+or Transform Whole Prop. Center Prop and Show Stickman Guide remain available.
+When the canvas
 is empty, it does not show the Arrange-only “Right-click to add a shape” hint.
 
 - Original vertices are filled circles.
@@ -133,7 +163,9 @@ private stickman Holding Preview. It does not add gameplay attachment or
 physics behavior to Sandbox Stage.
 
 Attach mode shows the Attach toolbar and the Anchors inspector. Shape editing
-controls are hidden, but the prop remains visible on the authoring canvas.
+controls are hidden, as are Normalize Size and Transform Whole Prop. Center Prop
+and Show Stickman Guide remain available, and the prop remains visible on the
+authoring canvas.
 
 ## Grid, Snap, and canvas navigation
 
@@ -491,10 +523,20 @@ Load dialogs report malformed or unsupported files through the Error dialog.
 
 ### Arrange transforms and context menu
 
+- [ ] Confirm the Arrange top bar order is Transform Whole Prop, Normalize Size,
+  Center Prop, then Show Stickman Guide.
 - [ ] Drag a selected shape body to translate it.
 - [ ] Drag the rotation dot and confirm the shape rotates continuously.
 - [ ] Hold Ctrl while rotating and confirm 15-degree stepping.
 - [ ] Drag each scale cross; hold Ctrl and confirm uniform scaling.
+- [ ] Enable Transform Whole Prop; drag visible geometry, the rotation dot, and
+  each corner cross, confirming that all shapes and editable anchors follow.
+- [ ] With Snap enabled, move the whole prop and confirm the move delta follows
+  the current grid; hold Ctrl while rotating and confirm 15-degree snapping.
+- [ ] Press Esc during a whole-prop gesture and confirm the preview cancels
+  without changing the document.
+- [ ] Switch to Reshape or Attach and confirm Transform Whole Prop and
+  Normalize Size are hidden while Center Prop and Show Stickman Guide remain.
 - [ ] Use Copy, right-click empty canvas, and Paste; confirm the pasted shape
   is offset and selected.
 - [ ] Test Flip Left-Right and Flip Up-Down.