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/pf-gpu/Cargo.toml

@@ -0,0 +1,28 @@
+# GPU vendor/adapter detection + selection + the live-session record, extracted into a leaf crate so
+# every subsystem crate (pf-encode, pf-capture, pf-vdisplay) can consult the selected GPU WITHOUT
+# depending on the orchestrator (plan §W6). Self-contained: reads config + writes the pref store.
+[package]
+name = "pf-gpu"
+version = "0.12.0"
+edition = "2021"
+license = "MIT OR Apache-2.0"
+description = "punktfunk host GPU vendor/adapter enumeration, selection preference, and active-session accounting."
+publish = false
+
+[dependencies]
+pf-host-config = { path = "../pf-host-config" }
+pf-paths = { path = "../pf-paths" }
+anyhow = "1"
+tracing = "0.1"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+[target.'cfg(windows)'.dependencies]
+windows = { version = "0.62", features = [
+    "Win32_Foundation",
+    "Win32_Graphics_Dxgi",
+    "Win32_Graphics_Dxgi_Common",
+] }
+
+[dev-dependencies]
+tempfile = "3"