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/vendor/ndk/src/lib.rs

@@ -0,0 +1,33 @@
+//! # Android NDK
+//!
+//! Bindings to the [Android NDK].
+//!
+//! [Android NDK]: https://developer.android.com/ndk/reference
+#![warn(
+    missing_debug_implementations,
+    rust_2018_idioms,
+    trivial_casts,
+    unused_qualifications
+)]
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
+
+pub mod asset;
+pub mod audio;
+pub mod bitmap;
+pub mod configuration;
+pub mod data_space;
+pub mod event;
+pub mod font;
+pub mod hardware_buffer;
+pub mod hardware_buffer_format;
+pub mod input_queue;
+pub mod looper;
+pub mod media;
+pub mod media_error;
+pub mod native_activity;
+pub mod native_window;
+pub mod shared_memory;
+pub mod surface_texture;
+pub mod sync;
+pub mod trace;
+mod utils;