fix: PID-based volume lock with state persistence and multiarch default path

Volume lock file now stores {pid}:{volume}:{muted} instead of "1":
- Allows recovery of saved volume state if the holder is SIGKILLed
- On stale lock detection (holder PID not alive), inherit saved volume state
  and take ownership — prevents permanent volume loss and infinite lockout

PAM module DEFAULT_PATH now baked in at build time via AHFAIL_LIBDIR env var
passed by Meson, fixing the wrong path on multiarch Debian/Ubuntu where libdir
is /usr/lib/x86_64-linux-gnu rather than /usr/lib.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/ahfail-pam/src/lib.rs

@@ -20,11 +20,13 @@ pub const PAM_DATA_REPLACE: c_int = 0x00000002;
 #[cfg(not(any(target_os = "linux", target_os = "macos")))]
 pub const PAM_DATA_REPLACE: c_int = 0x20000000u32 as i32;
 
-// Default install path for ahfail-display
+// Default install path for ahfail-display.
+// On Linux: built from AHFAIL_INSTALL_DIR emitted by build.rs, which reads AHFAIL_LIBDIR
+// passed by Meson — correct on multiarch systems (e.g. /usr/lib/x86_64-linux-gnu/ahfail).
 #[cfg(target_os = "macos")]
 const DEFAULT_PATH: &str = "/usr/local/lib/ahfail/ahfail-display";
 #[cfg(not(target_os = "macos"))]
-const DEFAULT_PATH: &str = "/usr/lib/ahfail/ahfail-display";
+const DEFAULT_PATH: &str = concat!(env!("AHFAIL_INSTALL_DIR"), "/ahfail-display");
 
 pub fn default_display_path() -> &'static str { DEFAULT_PATH }