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>

Cargo.lock

@@ -2808,6 +2808,13 @@ dependencies = [
  "pkg-config",
 ]
 
+[[package]]
+name = "pf-paths"
+version = "0.12.0"
+dependencies = [
+ "tracing",
+]
+
 [[package]]
 name = "pf-presenter"
 version = "0.12.0"
@@ -3122,6 +3129,7 @@ dependencies = [
  "opus",
  "parking_lot",
  "pf-driver-proto",
+ "pf-paths",
  "pipewire",
  "punktfunk-core",
  "pyrowave-sys",