fix(footnote): broaden preferred-reserve candidate filter for partial splits (SD-2656)

Vivienne's feedback on the rendering-fidelity PR called out footnotes
splitting across pages even when Word fits them on a single page.
Repro fixtures: 086 Carlsbad and b89cc7aa.

## Root cause

`isMandatoryOnlyFootnotePage` only flagged a page as a preferred-reserve
trial candidate when:
  actual_band ≈ mandatory  AND  lastAnchorRenderedLines <= 1

The scorer therefore never considered pages where the last anchor rendered
2+ lines and the remainder still spilled. These "partial split" cases are
the most common user-visible bug because the reader has to scroll to the
next page mid-footnote.

Repro on b89cc7aa.docx:
  page 16 — anchors=[4], mand=36, pref=82, actual=51, lastL=2, fn4 spilled
Repro on 086 Carlsbad:
  page 26 — anchors=[24], mand=42, pref=150, actual=116, lastL=5, fn24 spilled
  page 34 — anchors=[36], mand=42, pref=187, actual=61,  lastL=2, fn36 spilled
None of these entered the trial set.

## Fix

Adds `isSplitLastAnchorFootnotePage`: a page is also a candidate when its
last anchor appears in continuationOut AND the preferred reserve is
meaningfully bigger than current actual. `getPreferredReserveCandidates`
unions both predicates.

The scorer's accept criteria (no new cluster spills, no new mandatory-only
pages, bounded dead-reserve growth, candidate rendered lines improved)
stays unchanged — only the candidate filter widens.

## Verified

- b89cc7aa.docx: 4 split pages -> 1 split page (Vivienne's screenshot case
  on page 16 now renders fn4 fully on the anchor page).
- 086 Carlsbad.docx: 12 split pages unchanged (the remaining cases are
  multi-anchor with preferred deltas large enough that the scorer
  correctly rejects because of downstream cascade — same global
  protection as before).
- IT-923 (NVCA Model COI): 50 pages unchanged. No regression.
- 1253 layout-bridge tests pass (1 new test for the partial-split
  predicate, covering Vivienne's b89cc7aa page 16 and Carlsbad page 26
  patterns plus a non-spilled counter-example).
- 657 layout-engine, 1136 painter-dom pass.

Author: tupizz

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

@@ -81,13 +81,44 @@ export const isMandatoryOnlyFootnotePage = (
   );
 };
 
+/**
+ * SD-2656 (post-Vivienne-feedback): a page whose LAST anchor partially rendered
+ * but spilled to a later page. The user-visible bug is a footnote split across
+ * pages even when the preferred reserve would fit the whole anchor on the
+ * anchor page (Word does keep it together).
+ *
+ * The "mandatory-only" predicate catches first-line-only splits; this predicate
+ * catches partial splits (lastAnchorRenderedLines > 1 but the rest still spilled).
+ * Both feed into the same scorer trial. The scorer's accept criteria
+ * (no new cluster spills, no new mandatory-only pages, bounded dead-reserve
+ * growth, candidate rendered lines improved) still gates whether the bump
+ * actually lands.
+ */
+export const isSplitLastAnchorFootnotePage = (
+  ledger: FootnotePageLedger,
+  preferredDeltaThresholdPx = DEFAULT_PREFERRED_DELTA_THRESHOLD_PX,
+): boolean => {
+  if (ledger.anchorIds.length === 0) return false;
+  const lastAnchorId = ledger.anchorIds[ledger.anchorIds.length - 1];
+  const lastAnchorSpilled = ledger.continuationOut.some((entry) => entry.id === lastAnchorId);
+  if (!lastAnchorSpilled) return false;
+  return (
+    ledger.preferredReservePx - ledger.mandatoryReservePx > preferredDeltaThresholdPx &&
+    ledger.actualBandHeightPx < ledger.preferredReservePx - preferredDeltaThresholdPx
+  );
+};
+
 export const getPreferredReserveCandidates = (
   ledgers: FootnotePageLedger[],
   preferredDeltaThresholdPx = DEFAULT_PREFERRED_DELTA_THRESHOLD_PX,
   mandatoryOnlyTolerancePx = DEFAULT_MANDATORY_ONLY_TOLERANCE_PX,
 ): FootnotePreferredReserveCandidate[] => {
   return ledgers
-    .filter((ledger) => isMandatoryOnlyFootnotePage(ledger, preferredDeltaThresholdPx, mandatoryOnlyTolerancePx))
+    .filter(
+      (ledger) =>
+        isMandatoryOnlyFootnotePage(ledger, preferredDeltaThresholdPx, mandatoryOnlyTolerancePx) ||
+        isSplitLastAnchorFootnotePage(ledger, preferredDeltaThresholdPx),
+    )
     .map((ledger) => ({
       pageIndex: ledger.pageIndex,
       anchorIds: ledger.anchorIds.slice(),

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

@@ -72,6 +72,56 @@ describe('SD-2656 footnote preferred-reserve scorer', () => {
     ]);
   });
 
+  it('also flags pages where the last anchor partially rendered but spilled (Vivienne feedback)', () => {
+    // SD-2656: a page is also a candidate when the last anchor rendered >1 line
+    // yet still spilled to the next page. The legacy filter (lastAnchorRenderedLines<=1)
+    // missed these "partial split" cases reported by Vivienne — footnotes splitting
+    // across pages even when preferred reserve would fit them on the anchor page.
+    const ledgers = [
+      // mandatory-only first-line case (legacy candidate) — should still match.
+      makeLedger(0, {
+        anchorIds: ['1'],
+        mandatoryReservePx: 36,
+        preferredReservePx: 121,
+        actualBandHeightPx: 36,
+        lastAnchorRenderedLines: 1,
+        continuationOut: [{ id: '1', remainingRangeCount: 1, remainingHeightPx: 80 }],
+      }),
+      // Vivienne b89cc7aa page 16 pattern: single anchor [4], mand=36, pref=82,
+      // actual=51, lastL=2, fn4 spilled. Old filter missed this (lastL>1).
+      makeLedger(1, {
+        anchorIds: ['4'],
+        mandatoryReservePx: 36,
+        preferredReservePx: 82,
+        actualBandHeightPx: 51,
+        lastAnchorRenderedLines: 2,
+        continuationOut: [{ id: '4', remainingRangeCount: 1, remainingHeightPx: 30 }],
+      }),
+      // Carlsbad page 26 pattern: single anchor [24], mand=42, pref=150, actual=116,
+      // lastL=5, fn24 spilled. Old filter missed this.
+      makeLedger(2, {
+        anchorIds: ['24'],
+        mandatoryReservePx: 42,
+        preferredReservePx: 150,
+        actualBandHeightPx: 116,
+        lastAnchorRenderedLines: 5,
+        continuationOut: [{ id: '24', remainingRangeCount: 2, remainingHeightPx: 30 }],
+      }),
+      // Counter-example: last anchor rendered fully (no spill). Must NOT be a candidate.
+      makeLedger(3, {
+        anchorIds: ['5'],
+        mandatoryReservePx: 36,
+        preferredReservePx: 96,
+        actualBandHeightPx: 96,
+        lastAnchorRenderedLines: 5,
+        continuationOut: [],
+      }),
+    ];
+
+    const candidates = getPreferredReserveCandidates(ledgers).map((c) => c.pageIndex);
+    expect(candidates).toEqual([0, 1, 2]);
+  });
+
   it('summarizes only the candidate page window', () => {
     const ledgers = [
       makeLedger(0, {