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/lcs.ts

@@ -0,0 +1,26 @@
+/**
+ * Backward-filled LCS length table for sequences `a` and `b`: `dp[i][j]` is the
+ * length of the longest common subsequence of the suffixes `a[i:]` and `b[j:]`.
+ * O(n*m) time/space — fine for page block counts.
+ *
+ * Shared by the two-way block diff (`yjs-body-merge.diffBlocks`) and the
+ * three-way merge planner (`three-way-merge.lcsPairs`) so the (identical) table
+ * construction lives in ONE place; each caller does its own traceback over the
+ * returned table.
+ */
+export function buildLcsTable(a: string[], b: string[]): 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]);
+    }
+  }
+  return dp;
+}