feat(session): Skia console UI — overlay contract + stats OSD/capture HUD (phase 4a)

The §6.1 presenter↔console-UI contract lands: pf-presenter exposes its
device (SharedDevice) and composites at most one premultiplied-alpha
quad per frame (new overlay.frag + LOAD render pass over the swapchain;
zero cost while the overlay returns None). pf-console-ui implements it
with skia-safe on the shared VkDevice: DirectContext via the ash
dispatch chain, a ring of two offscreen render targets (one-frame-in-
flight safe), damage-driven redraws — the OSD re-renders at 1 Hz, the
hint on capture toggles, nothing per-frame. Skia never touches the
swapchain. The session binary carries it behind the default ui feature:
4.9 MB stripped without, 10 MB with (measured).

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

clients/linux-session/src/main.rs

@@ -152,6 +152,12 @@ mod session_main {
                 // This host's card carries the accent bar in the desktop client now.
                 trust::touch_last_used(&trust::hex(&fingerprint));
             })),
+            // The Skia console UI (stats OSD, capture HUD) — compiled out of the
+            // power-user build (`--no-default-features` drops the `ui` feature).
+            #[cfg(feature = "ui")]
+            overlay: Some(Box::new(pf_console_ui::SkiaOverlay::new())),
+            #[cfg(not(feature = "ui"))]
+            overlay: None,
         };
 
         let outcome = pf_presenter::run_session(opts, move |gamepad, native, force_software| {