refactor(git-sync): extract shared buildLcsTable for the two block diffs (PR #119 review)

The two-way block diff (yjs-body-merge.diffBlocks) and the three-way merge
planner (three-way-merge.lcsPairs) built the identical backward-filled LCS DP
table inline. Extract it to lcs.ts (buildLcsTable); each caller keeps its own
traceback. No behavior change — merge specs unchanged and green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/integrations/git-sync/services/three-way-merge.ts

@@ -19,21 +19,13 @@
  * instances (and the human's in-flight edits) in place.
  */
 
+import { buildLcsTable } from './lcs';
+
 /** Matched index pairs of the longest common subsequence of `a` and `b`. */
 function lcsPairs(a: string[], b: string[]): Array<[number, number]> {
   const n = a.length;
   const m = b.length;
-  const dp: number[][] = Array.from({ length: n + 1 }, () =>
-    new Array(m + 1).fill(0),
-  );
-  for (let i = n - 1; i >= 0; i--) {
-    for (let j = m - 1; j >= 0; j--) {
-      dp[i][j] =
-        a[i] === b[j]
-          ? dp[i + 1][j + 1] + 1
-          : Math.max(dp[i + 1][j], dp[i][j + 1]);
-    }
-  }
+  const dp = buildLcsTable(a, b);
   const pairs: Array<[number, number]> = [];
   let i = 0;
   let j = 0;