feat(clients/android): OnFrameRendered display stage — HUD headline becomes capture→displayed

The long-deferred Android display stage (design/stats-unification.md; plan 4.1 of
design/client-parity-and-network-resilience.md): AMediaCodec_setOnFrameRenderedCallback
(API 26, under the minSdk-28 floor ⇒ hard-linked via ndk-sys) reports SurfaceFlinger's
per-frame render timestamp, giving the HUD the spec's `display` = decoded→displayed term
and the directly-measured capture→displayed end-to-end headline on both decode loops.
Falls back per spec to the v1 capture→decoded endpoint on any window without render
callbacks (the platform may drop them under load), and to it permanently if registration
is refused.

- The render timestamp arrives on CLOCK_MONOTONIC; it's re-based onto CLOCK_REALTIME
  against monotonic-now at callback time, which also cancels the (batchable) callback
  delivery lag.
- The `ndk` crate exposes neither the callback nor the codec pointer needed to bind it
  raw, so the workspace pins `ndk` 0.9.0 to a vendored copy (clients/android/native/
  vendor/ndk) whose ONLY change makes MediaCodec::as_ptr public — the "as_ptr patch".
  Workspace-excluded so host builds never compile it; drop when upstream exposes either.
- nativeVideoStats grows to 26 doubles (22–25: dispValid, displayP50, e2eDispP50/P95;
  0–21 unchanged for older readers); StatsOverlay moves headline endpoint + equation
  together so the equation always tiles the headline interval.

Verified: host cargo check/test/clippy, aarch64-linux-android check/clippy, Kotlin
app+kit+tests compile, roborazzi HUD render shows the full 4-term equation. Device
verification rides plan 4.2's phone A/B.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/android/native/Cargo.toml

@@ -47,6 +47,12 @@ tracing = { version = "0.1", default-features = false, features = ["std", "log"]
 # (ANativeWindow_setFrameRate) is dlsym-resolved at runtime (see decode::try_set_frame_rate), not
 # linked, so the .so still loads on API 28/29.
 ndk = { version = "0.9", features = ["media", "audio", "nativewindow", "api-level-28"] }
+# Raw FFI for the one AMediaCodec entry point the `ndk` wrapper lacks:
+# `AMediaCodec_setOnFrameRenderedCallback` (API 26, under the minSdk-28 floor ⇒ hard-linked) — the
+# per-frame render-timestamp callback behind the HUD's `display` stage (see decode::DisplayTracker).
+# Reaching it needs the codec's raw pointer, which is why the workspace pins `ndk` to the vendored
+# copy whose only patch makes `MediaCodec::as_ptr` public (vendor/ndk, wired in the root Cargo.toml).
+ndk-sys = { version = "0.6", features = ["media"] }
 # setpriority/gettid to raise the decode thread toward URGENT_DISPLAY (see decode::boost_thread_priority).
 libc = "0.2"
 # Opus decode for the host→client audio plane (0xC9: 48 kHz stereo, 5 ms frames). Same crate the