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/Cargo.toml

@@ -14,6 +14,8 @@ punktfunk-core = { path = "../punktfunk-core", features = ["quic"] }
 pf-paths = { path = "../pf-paths" }
 # Process-wide host config global, extracted to a leaf crate (plan §W6).
 pf-host-config = { path = "../pf-host-config" }
+# GPU vendor/adapter detection + selection, extracted to a leaf crate (plan §W6).
+pf-gpu = { path = "../pf-gpu" }
 # M3 native control plane (the `punktfunk/1` QUIC handshake; data plane stays native-thread UDP).
 quinn = "0.11"
 anyhow = "1"