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/windows/install.rs

@@ -382,7 +382,7 @@ fn web_setup(args: &[String]) -> Result<()> {
     let app_dir =
         PathBuf::from(flag_val(args, "--app-dir").context("web setup: --app-dir <app> required")?);
     let pw_file = flag_val(args, "--password-file");
-    let data_dir = crate::gamestream::config_dir();
+    let data_dir = pf_paths::config_dir();
     std::fs::create_dir_all(&data_dir).ok();
     let pw_path = data_dir.join("web-password");
     let token_path = data_dir.join("mgmt-token");