feat: add SSE parser and in-flight request tracker

tests/inflight-tracker.test.ts

@@ -0,0 +1,50 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { InflightTracker, type FeedEvent } from "../src/lib/inflight-tracker";
+
+test("snapshot rebuilds counts for all in-flight requests", () => {
+  const tracker = new InflightTracker();
+  tracker.apply({
+    type: "inflight",
+    operation: "snapshot",
+    requests: [
+      { model: "A", id: "1" },
+      { model: "A", id: "2" },
+      { model: "B", id: "3" },
+    ],
+  });
+  assert.equal(tracker.count("A"), 2);
+  assert.equal(tracker.count("B"), 1);
+  assert.equal(tracker.count("C"), 0);
+});
+
+test("add and remove adjust per-model counts", () => {
+  const tracker = new InflightTracker();
+  tracker.apply({ type: "inflight", operation: "add", requests: [{ model: "A", id: "1" }] });
+  tracker.apply({ type: "inflight", operation: "add", requests: [{ model: "A", id: "2" }] });
+  assert.equal(tracker.count("A"), 2);
+  tracker.apply({ type: "inflight", operation: "remove", requests: [{ model: "A", id: "1" }] });
+  assert.equal(tracker.count("A"), 1);
+  tracker.apply({ type: "inflight", operation: "remove", requests: [{ model: "A", id: "2" }] });
+  assert.equal(tracker.count("A"), 0);
+  tracker.apply({ type: "inflight", operation: "remove", requests: [{ model: "A", id: "9" }] });
+  assert.equal(tracker.count("A"), 0);
+});
+
+test("modelStatus normalizes states", () => {
+  const tracker = new InflightTracker();
+  tracker.apply({
+    type: "modelStatus",
+    models: [
+      { id: "A", state: "ready" },
+      { id: "B", state: "loading" },
+      { id: "C", state: "stopped" },
+      { id: "D", state: "weird" },
+    ],
+  });
+  assert.equal(tracker.state("A"), "ready");
+  assert.equal(tracker.state("B"), "loading");
+  assert.equal(tracker.state("C"), "stopped");
+  assert.equal(tracker.state("D"), "stopped");
+  assert.equal(tracker.state("missing"), undefined);
+});