test+fix(telemetry): address PR review — dev-mode gate, session-storage dedupe, payload tests

Addresses review comments on #982:

- studio shouldTrack(): adds VITE_HYPERFRAMES_NO_TELEMETRY (mirrors CLI's
  HYPERFRAMES_NO_TELEMETRY) and import.meta.env.DEV gates so dev / CI
  studio builds don't pollute production telemetry. shouldTrack() is now
  exported for testability.
- App.tsx session dedupe: moves the once-per-session check from a useRef
  (which resets on HMR / remount) to sessionStorage via new
  hasFiredSessionStart / markSessionStartFired helpers in config.ts.
- studioRenderTelemetry.ts: documents why `workers` is intentionally
  omitted from emitStudioRenderError (studio renders don't accept a
  user-supplied worker count, so early failures genuinely don't know one).
- client.ts flush(): documents fire-and-forget no-retry design so future
  hands don't accidentally add retry logic that double-counts.

Tests:
- studioRenderTelemetry.test.ts (8 tests): perfPayload mapping for every
  RenderPerfSummary field, undefined-perf path, missing-extract path,
  zero-elapsed edge case, error event shape.
- studio/telemetry/events.test.ts (4 tests): pin event names
  (studio_session_start, studio_render_start) and payload shape.
- studio/telemetry/client.test.ts (9 tests): shouldTrack() returns false
  for non-phc_ key, opt-out, doNotTrack, build-time env, vite dev mode;
  memoization.

packages/studio/src/telemetry/client.test.ts

@@ -0,0 +1,100 @@
+// @vitest-environment happy-dom
+
+import { describe, expect, it, vi, beforeEach } from "vitest";
+
+// `shouldTrack()` reads `POSTHOG_API_KEY` from module-level const that's
+// evaluated at module load time, so changing `import.meta.env` after import
+// has no effect on the key. Each test resets module cache and re-imports.
+
+const OPT_OUT_KEY = "hyperframes-studio:telemetryDisabled";
+
+function setKey(value: string | undefined): void {
+  if (value === undefined) {
+    delete (import.meta.env as Record<string, unknown>).VITE_HYPERFRAMES_POSTHOG_KEY;
+  } else {
+    (import.meta.env as Record<string, unknown>).VITE_HYPERFRAMES_POSTHOG_KEY = value;
+  }
+}
+
+function setNoTelemetry(value: string | undefined): void {
+  if (value === undefined) {
+    delete (import.meta.env as Record<string, unknown>).VITE_HYPERFRAMES_NO_TELEMETRY;
+  } else {
+    (import.meta.env as Record<string, unknown>).VITE_HYPERFRAMES_NO_TELEMETRY = value;
+  }
+}
+
+function setDev(value: boolean): void {
+  (import.meta.env as { DEV: boolean }).DEV = value;
+}
+
+async function loadShouldTrack(): Promise<() => boolean> {
+  vi.resetModules();
+  const mod = await import("./client");
+  return mod.shouldTrack;
+}
+
+describe("studio client shouldTrack", () => {
+  beforeEach(() => {
+    setDev(false);
+    setKey("phc_test_key");
+    setNoTelemetry(undefined);
+    localStorage.clear();
+    vi.unstubAllGlobals();
+  });
+
+  it("returns true when key is configured, not in dev mode, and no opt-outs", async () => {
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(true);
+  });
+
+  it("returns false when API key does not start with phc_", async () => {
+    setKey("not_a_real_key");
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("returns false when API key is empty string", async () => {
+    setKey("");
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("returns false when user has opted out via localStorage", async () => {
+    localStorage.setItem(OPT_OUT_KEY, "1");
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("returns false when navigator.doNotTrack is '1'", async () => {
+    vi.stubGlobal("navigator", { ...navigator, doNotTrack: "1" });
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("returns false when VITE_HYPERFRAMES_NO_TELEMETRY=1 at build time", async () => {
+    setNoTelemetry("1");
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("returns false when VITE_HYPERFRAMES_NO_TELEMETRY='true'", async () => {
+    setNoTelemetry("true");
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("returns false in vite dev mode", async () => {
+    setDev(true);
+    const shouldTrack = await loadShouldTrack();
+    expect(shouldTrack()).toBe(false);
+  });
+
+  it("memoizes its decision after the first call", async () => {
+    const shouldTrack = await loadShouldTrack();
+    const first = shouldTrack();
+    // Flip an underlying input — memoized return must not change.
+    localStorage.setItem(OPT_OUT_KEY, "1");
+    expect(shouldTrack()).toBe(first);
+  });
+});