fix(footnote): allow extra dead-reserve when trial eliminates a split (SD-2656)

Second iteration on Vivienne's feedback. Previous candidate-filter fix
landed the b89cc7aa page 16 case but page 9 (anchors=[2,3], fn3 spilling)
still split because:

  * trial target=130 (full preferred) would eliminate the split
    (afterSplit=0, afterLines=1->6) but rejected for dead-reserve-bloat:
    148 px doc-wide growth > 128 px threshold;
  * trial target=125 then passed globally-safe but didn't fix the split
    (afterSplit=1) — the user-visible bug stayed.

The scorer was treating the dead-reserve threshold as absolute. But
eliminating a cluster split is a direct user-visible win that's worth
trading some downstream slack for.

## Fix

In `scoreFootnoteWindow`, double the window and document dead-reserve
allowance when the trial eliminates a cluster split in that scope:

  windowAllowance = eliminatesSplitInWindow ? base * 2 : base
  docAllowance    = eliminatesSplitInDoc    ? base * 2 : base

All other accept criteria (page count, new cluster-spills, new
mandatory-only pages, candidate rendered lines improved) stay strict.
Trials that just shift dead reserve without removing a split still hit
the original threshold.

## Verified

- b89cc7aa.docx: 4 split pages -> 0 split pages. Page 9 now renders fn3
  fully on the anchor page (actual=130 of preferred=130, lastL=6); page
  10 is body-only, matching Word.
- 086 Carlsbad.docx: 12 split pages unchanged. The remaining cases all
  reject for `page-count-grew` (bumping reserve pushes body to a new
  page) — that's a hard global guarantee unchanged by this fix.
- IT-923: pages 50 unchanged; splits 16 -> 15 (slight improvement).
- 1254 layout-bridge tests pass (1 new test for the relaxation, using
  b89cc7aa page 9 ledger values).

Author: tupizz

packages/layout-engine/layout-bridge/src/footnote-scorer.ts

@@ -365,13 +365,22 @@ export const scoreFootnoteWindow = (input: FootnoteWindowScoreInput): FootnoteWi
   if (hasNewId(afterDocumentDiagnostics.mandatoryOnlyAnchorIds, beforeDocumentDiagnostics.mandatoryOnlyAnchorIds)) {
     return { accept: false, reason: 'new-mandatory-only', before, after };
   }
-  if (after.deadReserveSum > before.deadReserveSum + deadReserveBloatThresholdPx) {
+  // SD-2656 (Vivienne feedback): a trial that ELIMINATES a cluster split is a
+  // direct user-visible win. Trade a larger dead-reserve growth for fewer
+  // footnotes splitting across pages. Without this relaxation the scorer
+  // accepts a smaller partial bump that improves mandatory-only count but
+  // leaves the split intact — the user sees no change.
+  const eliminatesSplitInWindow = beforeWindowDiagnostics.clusterSplitCount > afterWindowDiagnostics.clusterSplitCount;
+  const eliminatesSplitInDoc = beforeDocumentDiagnostics.clusterSplitCount > afterDocumentDiagnostics.clusterSplitCount;
+  const windowDeadAllowance = eliminatesSplitInWindow ? deadReserveBloatThresholdPx * 2 : deadReserveBloatThresholdPx;
+  const docDeadAllowance = eliminatesSplitInDoc
+    ? wholeDocumentDeadReserveBloatThresholdPx * 2
+    : wholeDocumentDeadReserveBloatThresholdPx;
+
+  if (after.deadReserveSum > before.deadReserveSum + windowDeadAllowance) {
     return { accept: false, reason: 'dead-reserve-bloat', before, after };
   }
-  if (
-    afterDocumentDiagnostics.deadReserveSum >
-    beforeDocumentDiagnostics.deadReserveSum + wholeDocumentDeadReserveBloatThresholdPx
-  ) {
+  if (afterDocumentDiagnostics.deadReserveSum > beforeDocumentDiagnostics.deadReserveSum + docDeadAllowance) {
     return { accept: false, reason: 'dead-reserve-bloat', before, after };
   }
   if (!candidateRenderedLinesImproved(before, after)) {

packages/layout-engine/layout-bridge/test/footnoteScorer.test.ts

@@ -235,6 +235,58 @@ describe('SD-2656 footnote preferred-reserve scorer', () => {
     expect(result.reason).toBe('page-count-grew');
   });
 
+  it('allows extra dead-reserve growth when the trial eliminates a cluster split (Vivienne feedback)', () => {
+    // SD-2656: a trial that removes a footnote-spanning split is a direct
+    // user-visible win, so the scorer trades up to 2x the normal dead-reserve
+    // growth allowance. Without this, the scorer rejected the full preferred
+    // bump on b89cc7aa page 9 (148 px doc-wide dead-reserve > 128 threshold)
+    // and accepted a smaller partial bump that left the split intact.
+    const beforeLedger = [
+      makeLedger(0, {
+        anchorIds: ['2', '3'],
+        mandatoryReservePx: 53,
+        preferredReservePx: 130,
+        actualBandHeightPx: 115,
+        deadReservePx: 0,
+        lastAnchorRenderedLines: 5,
+        continuationOut: [{ id: '3', remainingRangeCount: 1, remainingHeightPx: 30 }],
+      }),
+      makeLedger(1, {
+        anchorIds: [],
+        deadReservePx: 0,
+        continuationIn: [{ id: '3', remainingRangeCount: 1, remainingHeightPx: 30 }],
+      }),
+    ];
+    // After bumping page 0 to preferred: fn3 fully renders, split eliminated,
+    // but 148 px of dead reserve appears doc-wide (over the 128 default).
+    const afterLedger = [
+      makeLedger(0, {
+        anchorIds: ['2', '3'],
+        mandatoryReservePx: 53,
+        preferredReservePx: 130,
+        actualBandHeightPx: 130,
+        deadReservePx: 0,
+        lastAnchorRenderedLines: 7,
+      }),
+      makeLedger(1, {
+        anchorIds: [],
+        deadReservePx: 148,
+      }),
+    ];
+
+    const result = scoreFootnoteWindow({
+      beforeLayout: makeLayout(2, beforeLedger),
+      afterLayout: makeLayout(2, afterLedger),
+      candidatePageIndex: 0,
+      candidateAnchorId: '3',
+      beforeLedger,
+      afterLedger,
+    });
+
+    expect(result.accept).toBe(true);
+    expect(result.reason).toBe('globally-safe');
+  });
+
   it('accepts a direct candidate-line improvement without requiring unrelated pages to change', () => {
     const beforeLedger = [
       makeLedger(0, {