refactor(host/W6.1): extract GPU vendor/adapter detection into the pf-gpu leaf crate

Fourth de-coupling for the host crate carve (plan §W6.1 leaf). gpu.rs (inventory,
selection preference, active-session accounting — deps only pf-host-config + pf-paths, no
subsystem refs) moves to a new pf-gpu leaf so pf-encode/pf-capture/pf-vdisplay can consult
the selected GPU without an orchestrator edge. ~50 crate::gpu:: sites repoint to pf_gpu::;
the ~30 pub(crate) items become pub (crate API). assign_ids gets a macOS-only
allow(dead_code) (used only by the Linux/Windows enumerate arms).

Verified: Linux (home-worker-5) clippy -p pf-gpu -p punktfunk-host --all-targets
-D warnings + pf-gpu tests (12 pass); 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/vdisplay/windows/manager.rs

@@ -1462,7 +1462,7 @@ fn resolve_render_pin() -> Option<LUID> {
 /// not be compared with render-GPU picks.
 fn warn_if_pick_moved(mon: &Monitor) {
     let Some(pin) = mon.render_pin else { return };
-    let Some(sel) = crate::gpu::selected_gpu() else {
+    let Some(sel) = pf_gpu::selected_gpu() else {
         return;
     };
     let pick = sel.info.luid();