fix(memory): preserve BM25 relevance ordering (#33757, thanks @lsdcc01)

Land #33757 by @lsdcc01 without the unrelated dependency bump. Preserve negative FTS5 BM25 ordering in hybrid scoring and add changelog coverage for #5767.

Co-authored-by: 丁春才0668000523 <ding.chuncai1@xydigit.com>

src/memory/hybrid.test.ts

@@ -14,7 +14,18 @@ describe("memory hybrid helpers", () => {
     expect(bm25RankToScore(0)).toBeCloseTo(1);
     expect(bm25RankToScore(1)).toBeCloseTo(0.5);
     expect(bm25RankToScore(10)).toBeLessThan(bm25RankToScore(1));
-    expect(bm25RankToScore(-100)).toBeCloseTo(1);
+    expect(bm25RankToScore(-100)).toBeCloseTo(1, 1);
+  });
+
+  it("bm25RankToScore preserves FTS5 BM25 relevance ordering", () => {
+    const strongest = bm25RankToScore(-4.2);
+    const middle = bm25RankToScore(-2.1);
+    const weakest = bm25RankToScore(-0.5);
+
+    expect(strongest).toBeGreaterThan(middle);
+    expect(middle).toBeGreaterThan(weakest);
+    expect(strongest).not.toBe(middle);
+    expect(middle).not.toBe(weakest);
   });
 
   it("mergeHybridResults unions by id and combines weighted scores", async () => {

src/memory/hybrid.ts

@@ -44,8 +44,14 @@ export function buildFtsQuery(raw: string): string | null {
 }
 
 export function bm25RankToScore(rank: number): number {
-  const normalized = Number.isFinite(rank) ? Math.max(0, rank) : 999;
-  return 1 / (1 + normalized);
+  if (!Number.isFinite(rank)) {
+    return 1 / (1 + 999);
+  }
+  if (rank < 0) {
+    const relevance = -rank;
+    return relevance / (1 + relevance);
+  }
+  return 1 / (1 + rank);
 }
 
 export async function mergeHybridResults(params: {