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/Cargo.toml

@@ -12,9 +12,17 @@ repository.workspace = true
 name = "punktfunk-session"
 path = "src/main.rs"
 
+[features]
+default = ["ui"]
+# The Skia console UI (stats OSD, capture HUD, later the gamepad library). Dropping it
+# (`--no-default-features`) is the ~15 MB-smaller power-user build: same streaming,
+# stats on stdout only.
+ui = ["dep:pf-console-ui"]
+
 # Same Linux gating as the rest of the client stack; elsewhere this is a stub binary.
 [target.'cfg(target_os = "linux")'.dependencies]
 pf-presenter = { path = "../../crates/pf-presenter" }
+pf-console-ui = { path = "../../crates/pf-console-ui", optional = true }
 pf-client-core = { path = "../../crates/pf-client-core" }
 punktfunk-core = { path = "../../crates/punktfunk-core", features = ["quic"] }
 anyhow = "1"