feat(engine): cache probe Promise + log resolved mode + sync docs

Three follow-ups from Vai's staff-eng review:

1. Concurrent-probe race (real bug): the parallel coordinator runs N
   workers via Promise.all, so `--workers 4` on a no-GPU host fired 4
   simultaneous probe Chromes — each paying the same 240 ms launch cost.
   Cache the *Promise* (not the resolved value): first caller assigns
   the in-flight Promise, every other concurrent caller awaits the same
   one. Verified with a new test asserting all concurrent callers get
   the identical Promise reference.

2. Stale rendering.md (lines 23, 29): user-visible contract said
   "browser GPU enabled by default", which was wrong post-auto. Now
   describes the auto / hardware / software trichotomy explicitly.

3. Silent fallback: auto-mode produced no output, so a regression to
   "always falls back to software even with GPU present" would have
   been invisible in production logs. Added a single stderr line per
   process when the probe resolves: `[hyperframes] browserGpuMode auto
   → <mode> (<reason>)`. Cache hits don't re-log.

Verification:
- Engine 536/536 (incl. new concurrent-dedup test asserting Promise
  reference equality across simultaneous callers)
- CLI 256/256
- Format / lint / typecheck clean

packages/engine/src/services/browserManager.test.ts

@@ -103,6 +103,29 @@ describe("resolveBrowserGpuMode", () => {
     const third = await resolveBrowserGpuMode("hardware");
     expect(third).toBe("hardware");
   });
+
+  it("deduplicates concurrent auto-mode probes by caching the in-flight Promise", async () => {
+    // Parallel coordinator fires N workers via Promise.all — without Promise-
+    // level caching, a `--workers 4` render against a no-GPU host would launch
+    // 4 simultaneous probe Chromes. Verify all concurrent callers get the
+    // exact same Promise reference (proving the probe runs once, not N times).
+    const p1 = resolveBrowserGpuMode("auto", {
+      chromePath: "/definitely/not/a/real/chrome/binary",
+      browserTimeout: 2000,
+    });
+    const p2 = resolveBrowserGpuMode("auto", {
+      chromePath: "/definitely/not/a/real/chrome/binary",
+      browserTimeout: 2000,
+    });
+    const p3 = resolveBrowserGpuMode("auto", {
+      chromePath: "/definitely/not/a/real/chrome/binary",
+      browserTimeout: 2000,
+    });
+    expect(p1).toBe(p2);
+    expect(p2).toBe(p3);
+    const results = await Promise.all([p1, p2, p3]);
+    expect(results).toEqual(["software", "software", "software"]);
+  });
 });
 
 describe("forceReleaseBrowser", () => {