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>

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "crates/punktfunk-host",
     "crates/punktfunk-host/vendor/usbip-sim",
     "crates/punktfunk-tray",
+    "crates/pf-client-core",
     "crates/pf-driver-proto",
     "clients/probe",
     "clients/linux",