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/library/art.rs

@@ -19,10 +19,10 @@ fn art_cache() -> &'static std::sync::Mutex<std::collections::HashMap<String, Ar
 }
 
 /// The art cache lives in the canonical HOST config dir (`%ProgramData%\punktfunk` on Windows /
-/// `~/.config/punktfunk` on Linux — gamestream::config_dir, NOT the legacy XDG/HOME `config_dir`
+/// `~/.config/punktfunk` on Linux — `pf_paths::config_dir`, NOT the legacy XDG/HOME `config_dir`
 /// below that the custom store still uses).
 fn art_cache_path() -> PathBuf {
-    crate::gamestream::config_dir().join("library-art-cache.json")
+    pf_paths::config_dir().join("library-art-cache.json")
 }
 
 /// The cached art for a library id, if it has been resolved (positive or negative). `None` = not yet