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>

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "crates/pf-driver-proto",
     "crates/pf-paths",
     "crates/pf-host-config",
+    "crates/pf-gpu",
     "crates/pyrowave-sys",
     "clients/probe",
     "clients/linux",