refactor(host/W6.1): extract the config() global into the pf-host-config leaf crate

Third de-coupling for the host crate carve (plan §W6.1 leaf). HostConfig + the config()
OnceLock (config.rs, pure std, zero deps) move to a new pf-host-config leaf so every
subsystem crate (pf-encode/pf-capture/pf-vdisplay/pf-gpu) can read process config WITHOUT
depending on the orchestrator. 34 crate::config::config() call sites across 19 files
repoint to pf_host_config::config(). thread_qos stays in the host for now (it calls
session_tuning::on_hot_thread — its own leaf-ification rides the encode carve).

Granular-crate decision (supersedes the plan's single pf-media): split capture/encode/
vdisplay into separate crates rather than one broad crate — the capture↔encode cycle is
broken by a shared frame-types leaf, and vdisplay→encode (can_open_another_session) is a
legal one-way edge since encode never references vdisplay.

Verified: Linux (home-worker-5) clippy -p pf-host-config -p punktfunk-host --all-targets
-D warnings; Windows (192.168.1.158) clippy --features nvenc,amf-qsv --all-targets green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/inject.rs

@@ -124,7 +124,7 @@ pub fn default_backend() -> Backend {
         }
     }
     // An explicit compositor pick (set per connect / mid-stream) is the strongest signal.
-    let compositor = crate::config::config().compositor.clone();
+    let compositor = pf_host_config::config().compositor.clone();
     if let Some(c) = compositor.as_deref() {
         let c = c.trim();
         if c.eq_ignore_ascii_case("gamescope") {
@@ -176,7 +176,7 @@ pub(crate) use service::InjectorService;
 /// (`org.gnome.Mutter.RemoteDesktop`), the same direct API the Mutter video backend uses.
 #[cfg(target_os = "linux")]
 fn libei_ei_source() -> libei::EiSource {
-    let gnome = crate::config::config()
+    let gnome = pf_host_config::config()
         .compositor
         .as_deref()
         .is_some_and(|v| v.trim().eq_ignore_ascii_case("mutter"))