fix(worksheets): render operators last for proper layering

Use Typst's place() function to overlay + and − operators on top of
all other problem elements. This ensures operators are always visible
and properly layered over carry/borrow boxes, scratch work, and other
decorations.

Changes:
- Addition: wrap grid in box, use place() for + sign overlay
- Subtraction: extract operator to operatorOverlay.ts, use place()
- Both operators positioned at correct row using dy offset

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/app/create/worksheets/typstHelpers.ts

@@ -7,13 +7,13 @@
 // Import types for internal use
 import type { DisplayOptions } from './typstHelpers/shared/types'
 
+export { generatePlaceValueColors } from './typstHelpers/shared/colors'
+export { generateTypstHelpers } from './typstHelpers/shared/helpers'
 // Re-export everything from modular structure
 export type {
-  DisplayOptions,
   CellDimensions,
+  DisplayOptions,
 } from './typstHelpers/shared/types'
-export { generateTypstHelpers } from './typstHelpers/shared/helpers'
-export { generatePlaceValueColors } from './typstHelpers/shared/colors'
 export { generateSubtractionProblemStackFunction } from './typstHelpers/subtraction/problemStack'
 
 /**
@@ -103,12 +103,14 @@ export function generateProblemStackFunction(cellSize: number, maxDigits: number
     dir: ttb,
     spacing: 0pt,
     problem-number-display,
-    grid(
-      columns: column-list,
-      gutter: 0pt,
+    // Wrap grid in a box to enable place() overlay for operator
+    box[
+      #grid(
+        columns: column-list,
+        gutter: 0pt,
 
-      // Carry boxes row (one per place value, right to left)
-      [],  // Empty cell for + sign column
+        // Carry boxes row (one per place value, right to left)
+        [],  // Empty cell for + sign column
       ..for i in range(0, actual-digits).rev() {
         // DEBUG: Show which place values get carry boxes and why
         let show-carry = show-carries and i > 0
@@ -152,8 +154,8 @@ export function generateProblemStackFunction(cellSize: number, maxDigits: number
         ],)
       },
 
-      // Second addend row with + sign (right to left)
-      box(width: 0.5em, height: ${cellSizeIn})[#align(center + horizon)[#text(size: ${(cellSizePt * 0.8).toFixed(1)}pt)[+]]],
+      // Second addend row (operator sign rendered separately via place() for proper layering)
+      [],  // Empty cell for operator column (operator overlaid later)
       ..for i in range(0, actual-digits).rev() {
         let digit = b-digits.at(i)
         let place-color = place-colors.at(i)  // Dynamic color lookup by place value
@@ -240,7 +242,20 @@ export function generateProblemStackFunction(cellSize: number, maxDigits: number
           ],)
         }
       },
-    )
+      )
+      // Operator overlay - rendered last for proper layering
+      // Position: left edge, at second addend row vertical position
+      #place(
+        left + top,
+        dx: 0pt,
+        dy: ${cellSizeIn} * 2,  // Skip carry boxes row + first addend row
+        box(width: 0.5em, height: ${cellSizeIn})[
+          #align(center + horizon)[
+            #text(size: ${(cellSizePt * 0.8).toFixed(1)}pt)[+]
+          ]
+        ]
+      )
+    ]
   )
 }
 `

apps/web/src/app/create/worksheets/typstHelpers/subtraction/operatorOverlay.ts

@@ -0,0 +1,39 @@
+// Operator overlay for subtraction problems
+// Rendered last to ensure proper layering over all other elements
+
+import type { CellDimensions } from '../shared/types'
+
+/**
+ * Generate Typst code for the operator overlay
+ *
+ * The operator (− sign) is rendered using place() to overlay it on top of
+ * all other problem elements, ensuring proper layering regardless of
+ * borrow boxes, scratch work, or other decorations.
+ *
+ * @param cellDimensions - Cell sizing information
+ * @returns Typst code for operator overlay using place()
+ */
+export function generateOperatorOverlay(cellDimensions: CellDimensions): string {
+  const { cellSizeIn, cellSizePt } = cellDimensions
+
+  // The operator should be positioned at the subtrahend row level
+  // Borrow boxes row height + minuend row height = 2 * cellSize from top
+  // We position relative to the grid, so we need to account for:
+  // - Row 0: borrow boxes (height: cellSize)
+  // - Row 1: minuend row (height: cellSize)
+  // - Row 2: subtrahend row (where operator should appear)
+
+  return String.raw`
+    // Operator overlay - rendered last for proper layering
+    // Position: left edge, at subtrahend row vertical position
+    #place(
+      left + top,
+      dx: 0pt,
+      dy: ${cellSizeIn} * 2,  // Skip borrow boxes row + minuend row
+      box(width: 0.5em, height: ${cellSizeIn})[
+        #align(center + horizon)[
+          #text(size: ${(cellSizePt * 0.8).toFixed(1)}pt)[−]
+        ]
+      ]
+    )`
+}

apps/web/src/app/create/worksheets/typstHelpers/subtraction/problemStack.ts

@@ -3,10 +3,11 @@
 
 import { getPlaceValueColorNames } from '../shared/colors'
 import type { CellDimensions } from '../shared/types'
+import { generateAnswerBoxesRow, generateLineRow, generateTenFramesRow } from './answerRow'
 import { generateBorrowBoxesRow } from './borrowBoxes'
 import { generateMinuendRow } from './minuendRow'
+import { generateOperatorOverlay } from './operatorOverlay'
 import { generateSubtrahendRow } from './subtrahendRow'
-import { generateLineRow, generateTenFramesRow, generateAnswerBoxesRow } from './answerRow'
 
 /**
  * Generate the main subtraction problem stack function for Typst
@@ -115,9 +116,11 @@ export function generateSubtractionProblemStackFunction(
     dir: ttb,
     spacing: 0pt,
     problem-number-display,
-    grid(
-      columns: column-list,
-      gutter: 0pt,
+    // Wrap grid in a box to enable place() overlay for operator
+    box[
+      #grid(
+        columns: column-list,
+        gutter: 0pt,
 
 ${generateBorrowBoxesRow(cellDimensions)}
 
@@ -130,7 +133,9 @@ ${generateLineRow(cellDimensions)}
 ${generateTenFramesRow(cellDimensions)}
 
 ${generateAnswerBoxesRow(cellDimensions)}
-    )
+      )
+${generateOperatorOverlay(cellDimensions)}
+    ]
   )
 }
 `

apps/web/src/app/create/worksheets/typstHelpers/subtraction/subtrahendRow.ts

@@ -18,12 +18,8 @@ export function generateSubtrahendRow(cellDimensions: CellDimensions): string {
   const { cellSize, cellSizeIn, cellSizePt } = cellDimensions
 
   return String.raw`
-      // Subtrahend row with − sign
-      box(width: 0.5em, height: ${cellSizeIn})[
-        #align(center + horizon)[
-          #text(size: ${(cellSizePt * 0.8).toFixed(1)}pt)[−]
-        ]
-      ],
+      // Subtrahend row (operator sign rendered separately via place() for proper layering)
+      [],  // Empty cell for operator column (operator overlaid later)
       ..for i in range(0, grid-digits).rev() {
         let digit = s-digits.at(i)
         let place-color = place-colors.at(i)