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.lock

@@ -2808,6 +2808,20 @@ dependencies = [
  "pkg-config",
 ]
 
+[[package]]
+name = "pf-gpu"
+version = "0.12.0"
+dependencies = [
+ "anyhow",
+ "pf-host-config",
+ "pf-paths",
+ "serde",
+ "serde_json",
+ "tempfile",
+ "tracing",
+ "windows 0.62.2 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "pf-host-config"
 version = "0.12.0"
@@ -3133,6 +3147,7 @@ dependencies = [
  "opus",
  "parking_lot",
  "pf-driver-proto",
+ "pf-gpu",
  "pf-host-config",
  "pf-paths",
  "pipewire",