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>

crates/pf-presenter/src/lib.rs

@@ -18,6 +18,8 @@ pub mod input;
 #[cfg(target_os = "linux")]
 pub mod keymap_sdl;
 #[cfg(target_os = "linux")]
+pub mod overlay;
+#[cfg(target_os = "linux")]
 mod run;
 #[cfg(target_os = "linux")]
 pub mod vk;