fix(sdk/http): serialize concurrent writes to same path

Add per-path promise queue so rapid successive writes to the same
composition file cannot race at the server. Different paths still
write concurrently. Two new tests (RED→GREEN verified).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: vanceingalls

packages/sdk/src/adapters/http.test.ts

@@ -140,6 +140,7 @@ describe("flush()", () => {
     );
     const adapter = createHttpAdapter({ projectFilesUrl: BASE });
     void adapter.write("comp.html", "x"); // intentionally not awaited
+    await Promise.resolve(); // let path-queue microtask fire so doWrite starts
     let flushed = false;
     const flushDone = adapter.flush().then(() => {
       flushed = true;
@@ -167,6 +168,62 @@ describe("loadFrom()", () => {
   });
 });
 
+// ── write() — per-path serialization ─────────────────────────────────────────
+
+describe("write() — per-path serialization", () => {
+  it("serializes concurrent writes to the same path (second waits for first)", async () => {
+    const starts: number[] = [];
+    let resolveFirst!: () => void;
+    let callCount = 0;
+    vi.stubGlobal(
+      "fetch",
+      vi.fn().mockImplementation(async (_url: string, init?: RequestInit) => {
+        if (init?.method === "PUT") {
+          const n = ++callCount;
+          starts.push(n);
+          if (n === 1) await new Promise<void>((r) => (resolveFirst = r));
+        }
+        return { ok: true, status: 200, json: async () => ({}) };
+      }),
+    );
+    const adapter = createHttpAdapter({ projectFilesUrl: BASE });
+    const write1 = adapter.write("comp.html", "v1");
+    await Promise.resolve(); // let write1 start
+    const write2 = adapter.write("comp.html", "v2");
+    await Promise.resolve(); // let write2 attempt to start
+    expect(starts).toEqual([1]); // write2 has NOT started yet
+    resolveFirst();
+    await write1;
+    await write2;
+    expect(starts).toEqual([1, 2]); // write2 started only after write1 finished
+  });
+
+  it("does not block writes to different paths", async () => {
+    const starts: string[] = [];
+    let resolveFirst!: () => void;
+    let callCount = 0;
+    vi.stubGlobal(
+      "fetch",
+      vi.fn().mockImplementation(async (url: string, init?: RequestInit) => {
+        if (init?.method === "PUT") {
+          const n = ++callCount;
+          starts.push(`${n}:${url.split("/").pop()}`);
+          if (n === 1) await new Promise<void>((r) => (resolveFirst = r));
+        }
+        return { ok: true, status: 200, json: async () => ({}) };
+      }),
+    );
+    const adapter = createHttpAdapter({ projectFilesUrl: BASE });
+    const write1 = adapter.write("a.html", "v1");
+    await Promise.resolve();
+    void adapter.write("b.html", "v2"); // different path — must not wait for write1
+    await Promise.resolve();
+    expect(starts.length).toBe(2); // both started concurrently
+    resolveFirst();
+    await write1;
+  });
+});
+
 // ── on() / unsubscribe ────────────────────────────────────────────────────────
 
 describe("on() / unsubscribe", () => {

packages/sdk/src/adapters/http.ts

@@ -13,6 +13,7 @@ class HttpAdapter implements PersistAdapter {
   private readonly baseUrl: string;
   private readonly errorListeners: Array<(e: PersistErrorEvent) => void> = [];
   private readonly inflightWrites = new Set<Promise<void>>();
+  private readonly pathQueues = new Map<string, Promise<void>>();
 
   constructor(opts: HttpAdapterOptions) {
     this.baseUrl = opts.projectFilesUrl;
@@ -27,7 +28,12 @@ class HttpAdapter implements PersistAdapter {
   }
 
   async write(path: string, content: string): Promise<void> {
-    const p = this.doWrite(path, content);
+    const prev = this.pathQueues.get(path) ?? Promise.resolve();
+    const p = prev.then(() => this.doWrite(path, content));
+    this.pathQueues.set(
+      path,
+      p.catch(() => {}),
+    );
     this.inflightWrites.add(p);
     try {
       await p;