feat: support paragraph between borders (w:pBdr/w:between) (#2074)

Flow `w:between` borders through the rendering pipeline so consecutive
paragraphs sharing the same border definition display a horizontal
separator line between them.

- Add `between` to `ParagraphBorders` contract type
- Include `between` in `normalizeParagraphBorders` side iteration
- Add `bw:` segment to `hashParagraphBorders` in both hash util files
- Add `computeBetweenBorderFlags` to pre-compute which fragments need
  the between border, following the `computeSdtBoundaries` pattern
- Render between border as CSS `border-bottom` on decoration overlay
- Handle header/footer sections, page splits, and patch invalidation
- Add 55+ tests covering edge cases

Author: gpardhivvarma

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

@@ -1097,6 +1097,7 @@ export type ParagraphBorders = {
   right?: ParagraphBorder;
   bottom?: ParagraphBorder;
   left?: ParagraphBorder;
+  between?: ParagraphBorder;
 };
 
 export type ParagraphShading = {

packages/layout-engine/layout-bridge/src/paragraph-hash-utils.ts

@@ -38,6 +38,7 @@ export const hashParagraphBorders = (borders: ParagraphBorders): string => {
   if (borders.right) parts.push(`r:[${hashParagraphBorder(borders.right)}]`);
   if (borders.bottom) parts.push(`b:[${hashParagraphBorder(borders.bottom)}]`);
   if (borders.left) parts.push(`l:[${hashParagraphBorder(borders.left)}]`);
+  if (borders.between) parts.push(`bw:[${hashParagraphBorder(borders.between)}]`);
   return parts.join(';');
 };
 

packages/layout-engine/layout-bridge/test/paragraph-hash-utils.test.ts

@@ -5,8 +5,15 @@
  */
 
 import { describe, it, expect } from 'vitest';
-import { hashBorderSpec, hashTableBorderValue, hashTableBorders, hashCellBorders } from '../src/paragraph-hash-utils';
-import type { BorderSpec, TableBorders, CellBorders } from '@superdoc/contracts';
+import {
+  hashBorderSpec,
+  hashTableBorderValue,
+  hashTableBorders,
+  hashCellBorders,
+  hashParagraphBorders,
+  hashParagraphAttrs,
+} from '../src/paragraph-hash-utils';
+import type { BorderSpec, TableBorders, CellBorders, ParagraphBorders, ParagraphAttrs } from '@superdoc/contracts';
 
 describe('hashBorderSpec', () => {
   it('produces deterministic hash for same border properties', () => {
@@ -391,3 +398,87 @@ describe('hashCellBorders', () => {
     expect(hash).toContain('sp:2');
   });
 });
+
+describe('hashParagraphBorders', () => {
+  it('includes between border in hash with bw: prefix', () => {
+    const borders: ParagraphBorders = {
+      top: { style: 'solid', width: 1, color: '#000' },
+      between: { style: 'solid', width: 2, color: '#FF0000' },
+    };
+    const hash = hashParagraphBorders(borders);
+    expect(hash).toContain('t:[');
+    expect(hash).toContain('bw:[');
+    expect(hash).toContain('w:2');
+  });
+
+  it('produces different hashes with and without between', () => {
+    const with_: ParagraphBorders = {
+      top: { style: 'solid', width: 1 },
+      between: { style: 'solid', width: 1 },
+    };
+    const without_: ParagraphBorders = {
+      top: { style: 'solid', width: 1 },
+    };
+    expect(hashParagraphBorders(with_)).not.toBe(hashParagraphBorders(without_));
+  });
+
+  it('does not include bw: when between is undefined', () => {
+    const borders: ParagraphBorders = {
+      top: { style: 'solid', width: 1 },
+      bottom: { style: 'solid', width: 1 },
+    };
+    expect(hashParagraphBorders(borders)).not.toContain('bw:');
+  });
+
+  it('places bw: after l: in hash output', () => {
+    const borders: ParagraphBorders = {
+      left: { style: 'solid', width: 1 },
+      between: { style: 'solid', width: 1 },
+    };
+    const hash = hashParagraphBorders(borders);
+    expect(hash.indexOf('l:[')).toBeLessThan(hash.indexOf('bw:['));
+  });
+});
+
+describe('hashParagraphAttrs', () => {
+  it('includes between border in attrs hash via borders', () => {
+    const attrs: ParagraphAttrs = {
+      borders: {
+        top: { style: 'solid', width: 1 },
+        between: { style: 'solid', width: 2, color: '#F00' },
+      },
+    };
+    const hash = hashParagraphAttrs(attrs);
+    expect(hash).toContain('br:');
+    expect(hash).toContain('bw:[');
+  });
+
+  it('produces different hashes when between border changes', () => {
+    const attrs1: ParagraphAttrs = {
+      borders: {
+        top: { style: 'solid', width: 1 },
+        between: { style: 'solid', width: 1 },
+      },
+    };
+    const attrs2: ParagraphAttrs = {
+      borders: {
+        top: { style: 'solid', width: 1 },
+        between: { style: 'dashed', width: 2 },
+      },
+    };
+    expect(hashParagraphAttrs(attrs1)).not.toBe(hashParagraphAttrs(attrs2));
+  });
+
+  it('produces different hashes when between border is added', () => {
+    const withoutBetween: ParagraphAttrs = {
+      borders: { top: { style: 'solid', width: 1 } },
+    };
+    const withBetween: ParagraphAttrs = {
+      borders: {
+        top: { style: 'solid', width: 1 },
+        between: { style: 'solid', width: 1 },
+      },
+    };
+    expect(hashParagraphAttrs(withoutBetween)).not.toBe(hashParagraphAttrs(withBetween));
+  });
+});