refactor(client): extract UI-agnostic plumbing into pf-client-core (phase 0)

Session pump, FFmpeg decode, PipeWire audio, SDL3 gamepads, keymap, trust
store, mDNS discovery, library client and Wake-on-LAN move verbatim from
clients/linux into crates/pf-client-core, shared with the upcoming Vulkan
session binary (punktfunk-planning: linux-client-rearchitecture.md).
The GTK client re-exports them at the crate root so every existing
crate::-path keeps resolving; its manifest drops the moved-only deps.

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

crates/pf-client-core/src/lib.rs

@@ -0,0 +1,26 @@
+//! Shared, UI-agnostic Linux-client plumbing, extracted verbatim from the GTK client
+//! (design: punktfunk-planning `linux-client-rearchitecture.md`, Phase 0) so the desktop
+//! shell and the Vulkan session binary build on one implementation.
+//!
+//! Nothing here may depend on a UI toolkit: the presenter contract is `session`'s
+//! channels (`SessionHandle`) and `video`'s `DecodedImage` (RGBA bytes or dmabuf fds +
+//! plane layout) — how frames reach the screen is the consumer's business.
+
+#[cfg(target_os = "linux")]
+pub mod audio;
+#[cfg(target_os = "linux")]
+pub mod discovery;
+#[cfg(target_os = "linux")]
+pub mod gamepad;
+#[cfg(target_os = "linux")]
+pub mod keymap;
+#[cfg(target_os = "linux")]
+pub mod library;
+#[cfg(target_os = "linux")]
+pub mod session;
+#[cfg(target_os = "linux")]
+pub mod trust;
+#[cfg(target_os = "linux")]
+pub mod video;
+
+pub mod wol;