refactor(host/W6.1): extract secret/config-dir helpers into the pf-paths leaf crate

Second de-coupling for the host crate carve (plan §W6.1 leaf). config_dir /
create_private_dir / write_secret_file (+ the Windows DACL helpers) were pub(crate) in the
gamestream junk drawer, yet consumed by vdisplay, stats, gpu, library, mgmt_token,
native_pairing and the Windows service — many of which become pf-media / pf-vdisplay, for
which crate::gamestream would be an illegal upward edge. New leaf crate pf-paths (pure std
+ tracing) owns them; ~40 call sites across 14 files repoint to pf_paths::. gamestream
keeps only its own concerns.

Verified: Linux (home-worker-5) clippy -p pf-paths -p punktfunk-host --all-targets
-D warnings + tests (347 pass, incl. secrets_are_written_owner_only); 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/gamestream/apps.rs

@@ -28,7 +28,7 @@ pub struct AppEntry {
 
 fn config_path() -> Option<std::path::PathBuf> {
     // `config_dir()` resolves XDG/HOME on Linux and %APPDATA% on Windows (no HOME needed).
-    Some(super::config_dir().join("apps.json"))
+    Some(pf_paths::config_dir().join("apps.json"))
 }
 
 fn parse_compositor(s: &str) -> Option<crate::vdisplay::Compositor> {