fix(columns): getColumnGeometry honors count when widths is absent (SD-2629)

The balancing migration (bd2ccf1) builds its geometry input directly from
columnCount/columnGap/columnWidth and only attaches a widths array in
explicit mode. In equal mode it passed {count, gap, width} with no widths,
and getColumnGeometry fell back to a single [width] column - so every column
index past 0 clamped onto column 0 and balanced content stacked on the left
margin (col2 x 432 -> 96 on the last page of equal-width multi-column docs).

A geometry must have exactly count columns. Expand the scalar width to count
equal columns when no widths array is present rather than collapsing to one.
normalizeColumnLayout already emits one width per column, so the normal
engine path is unchanged; this only hardens the hand-built case. The layout
corpus caught it (9 equal-width multi-column docs regressed; all restored).

Author: caio-pizzol

packages/layout-engine/contracts/src/column-layout.test.ts

@@ -203,6 +203,18 @@ describe('getColumnGeometry + geometry helpers (SD-2629)', () => {
     expect(geom[0].gapAfter).toBe(999);
     expect(geom[1].x).toBe(300 + 999);
   });
+
+  it('expands an equal-mode layout with no widths array to `count` columns (SD-2629 regression)', () => {
+    // A hand-built equal-mode layout (column-balancing) carries only the scalar `width`, no widths
+    // array. Geometry must still yield `count` columns; collapsing to a single column mapped every
+    // index past 0 onto column 0's x, stacking balanced multi-column content on the left margin.
+    const geom = getColumnGeometry({ count: 2, gap: 48, width: 288 });
+    expect(geom).toEqual([
+      { index: 0, x: 0, width: 288, gapAfter: 48 },
+      { index: 1, x: 336, width: 288, gapAfter: 0 },
+    ]);
+    expect(getColumnX(geom, 1, 96)).toBe(432);
+  });
 });
 
 describe('columnLayoutsEqual', () => {

packages/layout-engine/contracts/src/column-layout.ts

@@ -182,8 +182,16 @@ export function normalizeColumnLayout(
  * widths and per-column `gaps`, falling back to the uniform gap when no per-column gaps exist.
  */
 export function getColumnGeometry(normalized: NormalizedColumnLayout): ColumnGeometry[] {
+  // A geometry must have exactly `count` columns. normalizeColumnLayout always emits one width per
+  // column, but a hand-built equal-mode layout may carry only the scalar `width` with no widths array
+  // (e.g. column-balancing constructs its input directly). Expand that to `count` equal columns
+  // instead of collapsing to a single [width] column, which would map every column index past 0 onto
+  // column 0's x and stack later columns on the left margin. (SD-2629)
+  const count = Number.isFinite(normalized.count) ? Math.max(1, Math.floor(normalized.count)) : 1;
   const widths =
-    Array.isArray(normalized.widths) && normalized.widths.length > 0 ? normalized.widths : [normalized.width];
+    Array.isArray(normalized.widths) && normalized.widths.length > 0
+      ? normalized.widths
+      : new Array(count).fill(normalized.width);
   return buildColumnGeometry(widths, normalized.gap, Boolean(normalized.withSeparator), normalized.gaps);
 }