Add a shared differential-style list and index pivot formats into it

src/types/pivotTables/PivotFormat.ts

@@ -1,4 +1,4 @@
-import type { Style } from '../styles/index.ts';
+import type { integer } from '../integer.ts';
 import type { PivotArea } from './PivotArea.ts';
 
 /**
@@ -8,7 +8,7 @@ import type { PivotArea } from './PivotArea.ts';
  * format targets with an action indicating whether formatting is applied or blanked.
  *
  * Formats are the durable representation of user formatting on pivot output: a consumer
- * re-resolves each format's area against the current layout and overlays its {@link style} on
+ * re-resolves each format's area against the current layout and overlays its differential style on
  * every repaint, so the formatting follows the targeted region (an item's data cells, the grand
  * total row, ...) when the pivot reshapes, rather than sticking to cell coordinates.
  *
@@ -24,9 +24,11 @@ export type PivotFormat = {
   /** The pivot table region this format applies to. */
   pivotArea: PivotArea;
   /**
-   * The formatting to overlay on the area's cells. Only the properties present apply; everything
-   * else on a cell is left as-is (the differential-format semantics of OOXML's `dxf`).
-   * Absent for `action: 'blank'`.
+   * Index of the differential style to overlay on the area's cells, into
+   * {@link Workbook.diffStyles}. Only the properties present in that style apply; everything else
+   * on a cell is left as-is. Absent for `action: 'blank'`.
+   *
+   * @see {@link Workbook.diffStyles}
    */
-  style?: Style;
+  diffStyleId?: integer;
 };

src/types/pivotTables/PivotTable.ts

@@ -92,9 +92,9 @@ export type PivotTable = {
   /** Presentation style for the pivot table. */
   style?: PivotTableStyle;
   /**
-   * Custom formatting applied to specific regions of the pivot table. Each format carries its
-   * differential style inline (see {@link PivotFormat.style}); there is no separate dxf table
-   * in JSF.
+   * Custom formatting applied to specific regions of the pivot table. Each format references a
+   * shared differential style by index (see {@link PivotFormat.diffStyleId} and
+   * {@link Workbook.diffStyles}).
    */
   formats?: PivotFormat[];
   // conditionalFormats is omitted for now: the conditional formatting model is not finalized.

src/types/workbooks/Workbook.ts

@@ -31,6 +31,22 @@ export type Workbook = {
   calculationProperties?: CalcProps;
   /** Styles for cells in the workbook. */
   styles?: Style[];
+  /**
+   * Differential styles: partial style overrides referenced by index from features that overlay
+   * formatting on top of a cell's own style rather than replacing it (pivot table formats, custom
+   * table styles, and future dynamic/conditional styles).
+   *
+   * This is the differential-overlay analog of {@link Workbook.styles}: where {@link Cell.s}
+   * indexes a complete cell style in `styles`, an overlay's `diffStyleId` indexes a partial
+   * override here. Only the properties present in an entry apply; everything else on the target
+   * cell is left as-is.
+   *
+   * Each entry reuses the {@link Style} type (already all-optional), but is interpreted
+   * differentially: an absent property means "leave unchanged", not "use the default".
+   *
+   * @see {@link PivotFormat.diffStyleId}
+   */
+  diffStyles?: Style[];
   /** Named cell style definitions (e.g. "Normal", "Heading 1"), keyed by style name. */
   namedStyles?: Record<string, NamedStyle>;
   /** External cells referenced by the workbook. An external cell is a cell in another workbook. */