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/README.md

@@ -23,6 +23,11 @@ In-stream keys match the desktop client: click captures input (Ctrl+Alt+Shift+Q
 releases), Ctrl+Alt+Shift+D disconnects, F11 toggles fullscreen; the controller escape
 chord (L1+R1+Start+Select, hold to disconnect) works the same.
 
+The default build carries the Skia console UI (`ui` feature): the stats OSD and capture
+hint render in-window (Ctrl+Alt+Shift+S toggles both the OSD and the stdout mirror).
+`--no-default-features` is the ~5 MB power-user build — same streaming, stats on stdout
+only, no Skia anywhere in the dependency tree.
+
 Decode follows the Settings preference: VAAPI frames import zero-copy into Vulkan
 (per-plane dmabuf + the stream's CICP-driven CSC shader); boxes whose driver can't
 import (NVIDIA proprietary by design) fall back to software decode automatically.