fix(camera): selectable snapshot stream + retry transient 503 Device Busy

A Hikvision DS-2CD1047G3H-LIU returned HTTP 503 (statusCode 2 / deviceBusy)
on EVERY main-stream snapshot — its main encoder is persistently saturated.
Probed on hardware: channels/101/picture → 503 on 5 consecutive tries, while
channels/102/picture (sub stream) → 200 clean JPEG every time. A retry loop
can't fix a persistent busy; the real fix is stream selection.

- Add a `stream` config field to the Hikvision driver (1=main, default for
  back-compat; 2=sub). ISAPI channel id is <channel><stream> (101 main, 102 sub).
  Verified live: setting the G3H to Sub flips its status degraded→ready (14.7KB
  JPEG in ~87ms).
- captureSnapshot also retries the TRANSIENT case (503/500, linear backoff
  250/500/750ms ×4) then fails naming it "(device busy)"; does NOT retry 401/404
  (config errors won't self-heal). Complements captureSnapshotShared (concurrent
  de-dup). healthCheck still reports a live 503 as degraded (surfaces a saturated
  main stream rather than hiding it).

Tests: camera.test.ts (10) — retry behaviour + main/sub path selection.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

packages/devices/src/drivers/camera.test.ts

@@ -0,0 +1,115 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import type { DigestGetResult } from "./http-digest.js";
+
+// The HTTP layer is mocked so the camera driver's RETRY logic is tested without a
+// network. Hikvision returns 503 "Device Busy" (sometimes 500) transiently when its
+// snapshot encoder is occupied — captureSnapshot must retry those and succeed, but
+// fail FAST on a config error (401 auth / 404 path). See camera.ts.
+
+const digestGet = vi.fn<(...a: unknown[]) => Promise<DigestGetResult>>();
+vi.mock("./http-digest.js", () => ({ digestGet: (...a: unknown[]) => digestGet(...a) }));
+
+// Import the driver AFTER the mock is registered.
+const { hikvisionDriver } = await import("./camera.js");
+
+function reply(status: number, body = "jpeg-bytes"): DigestGetResult {
+  return { status, contentType: "image/jpeg", body: Buffer.from(body) };
+}
+
+function makeCamera() {
+  return hikvisionDriver.create({ host: "10.0.10.12", port: 80, username: "admin", password: "x", channel: 1 });
+}
+
+beforeEach(() => {
+  digestGet.mockReset();
+  vi.useFakeTimers();
+});
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe("hikvision captureSnapshot — 503 Device Busy retry", () => {
+  it("retries a transient 503 and succeeds", async () => {
+    digestGet
+      .mockResolvedValueOnce(reply(503))
+      .mockResolvedValueOnce(reply(503))
+      .mockResolvedValueOnce(reply(200, "the-frame"));
+    const cam = makeCamera();
+    const p = cam.captureSnapshot({ direction: "entry" });
+    await vi.runAllTimersAsync(); // let the backoff sleeps fire
+    const shot = await p;
+    expect(shot.bytes.toString()).toBe("the-frame");
+    expect(digestGet).toHaveBeenCalledTimes(3); // 503, 503, 200
+  });
+
+  it("also retries a transient 500", async () => {
+    digestGet.mockResolvedValueOnce(reply(500)).mockResolvedValueOnce(reply(200));
+    const cam = makeCamera();
+    const p = cam.captureSnapshot({ direction: "entry" });
+    await vi.runAllTimersAsync();
+    await p;
+    expect(digestGet).toHaveBeenCalledTimes(2);
+  });
+
+  it("gives up after the attempt cap, naming it 'device busy'", async () => {
+    digestGet.mockResolvedValue(reply(503)); // always busy
+    const cam = makeCamera();
+    // Attach the rejection assertion BEFORE flushing timers so the rejection always
+    // has a handler (no unhandled-rejection noise), then drive the backoff sleeps.
+    const assertion = expect(cam.captureSnapshot({ direction: "entry" })).rejects.toThrow(/HTTP 503 \(device busy\)/);
+    await vi.runAllTimersAsync();
+    await assertion;
+    expect(digestGet).toHaveBeenCalledTimes(4); // SNAPSHOT_MAX_ATTEMPTS
+  });
+
+  it("does NOT retry a 401 (auth error self-won't-heal) — fails fast", async () => {
+    digestGet.mockResolvedValue(reply(401));
+    const cam = makeCamera();
+    const assertion = expect(cam.captureSnapshot({ direction: "entry" })).rejects.toThrow(/HTTP 401/);
+    await vi.runAllTimersAsync();
+    await assertion;
+    expect(digestGet).toHaveBeenCalledTimes(1); // no retry
+  });
+
+  it("does NOT retry a 404 (wrong path/channel) — fails fast", async () => {
+    digestGet.mockResolvedValue(reply(404));
+    const cam = makeCamera();
+    const assertion = expect(cam.captureSnapshot({ direction: "entry" })).rejects.toThrow(/HTTP 404/);
+    await vi.runAllTimersAsync();
+    await assertion;
+    expect(digestGet).toHaveBeenCalledTimes(1);
+  });
+
+  it("succeeds first try with no retry on a clean 200", async () => {
+    digestGet.mockResolvedValue(reply(200));
+    const cam = makeCamera();
+    const shot = await cam.captureSnapshot({ direction: "entry" });
+    expect(shot.contentType).toBe("image/jpeg");
+    expect(digestGet).toHaveBeenCalledTimes(1);
+  });
+});
+
+describe("hikvision snapshot stream selection (main vs sub)", () => {
+  function pathFor(config: Record<string, unknown>): string {
+    digestGet.mockReset();
+    digestGet.mockResolvedValue(reply(200));
+    hikvisionDriver.create(config as never).captureSnapshot({ direction: "entry" });
+    return String((digestGet.mock.calls[0]![0] as { path: string }).path);
+  }
+
+  it("defaults to the MAIN stream (…/channels/101/picture) — back-compat", () => {
+    expect(pathFor({ host: "1.2.3.4", channel: 1 })).toBe("/ISAPI/Streaming/channels/101/picture");
+  });
+
+  it("stream=2 selects the SUB stream (…/channels/102/picture) — the G3H 503 fix", () => {
+    expect(pathFor({ host: "1.2.3.4", channel: 1, stream: 2 })).toBe("/ISAPI/Streaming/channels/102/picture");
+  });
+
+  it("honours the channel number with the stream (ch2 sub = 202)", () => {
+    expect(pathFor({ host: "1.2.3.4", channel: 2, stream: 2 })).toBe("/ISAPI/Streaming/channels/202/picture");
+  });
+
+  it("an invalid stream falls back to main (1)", () => {
+    expect(pathFor({ host: "1.2.3.4", channel: 1, stream: 9 })).toBe("/ISAPI/Streaming/channels/101/picture");
+  });
+});