fix(embedder): update LRU insertion order when re-setting existing key

Issue #598 Phase 3: When an existing cache key is re-set, explicitly
delete it first to maintain correct LRU ordering. Without this, the
insertion order stays stale and the LRU eviction removes the wrong entry.

Also adds unit test for LRU semantics.

Author: jlin53882

src/embedder.ts

@@ -69,6 +69,10 @@ class EmbeddingCache {
 
   set(text: string, task: string | undefined, vector: number[]): void {
     const k = this.key(text, task);
+    // If key already exists, delete to update insertion order for correct LRU semantics
+    if (this.cache.has(k)) {
+      this.cache.delete(k);
+    }
     // When cache is full, run TTL eviction first (removes expired + oldest).
     // This prevents unbounded growth from stale entries while keeping writes O(1).
     if (this.cache.size >= this.maxSize) {

test/embedder-lru.test.mjs

@@ -0,0 +1,32 @@
+/**
+ * Issue #598 Phase 3 - EmbeddingCache LRU Semantics Test
+ * Tests that re-setting an existing key updates its LRU position.
+ */
+
+import { describe, it } from "node:test";
+import assert from "node:assert";
+
+const jiti = require("jiti")(__filename);
+
+describe("EmbeddingCache LRU semantics", () => {
+  it("updates insertion order when re-setting existing key", async () => {
+    const { Embedder } = jiti("../src/embedder.ts");
+
+    const embedder = new Embedder({});
+    const cache = embedder["cache"];
+
+    // Set two keys
+    cache.set("key1", undefined, [1, 0, 0]);
+    cache.set("key2", undefined, [0, 1, 0]);
+
+    // Re-set key1 with different value (should move to most recent)
+    cache.set("key1", undefined, [1, 1, 0]);
+
+    // Evict one - should evict key2 (oldest), not key1 (which was re-set)
+    cache.set("key3", undefined, [0, 0, 1]);
+
+    assert.strictEqual(cache.cache.has("key1"), true, "key1 should remain after re-set");
+    assert.strictEqual(cache.cache.has("key2"), false, "key2 should be evicted (oldest)");
+    assert.strictEqual(cache.cache.has("key3"), true, "key3 should be added");
+  });
+});