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>

meson.build

@@ -38,7 +38,8 @@ pam_cargo = custom_target(
   output: ['libahfail_pam.so'],
   command: [
     'sh', '-c',
-    'cargo build --release -p ahfail-pam --target-dir "@OUTDIR@/target-pam" && cp "@OUTDIR@/target-pam/release/libahfail_pam.so" "@OUTPUT@"'
+    # Pass libdir so build.rs emits the correct AHFAIL_INSTALL_DIR (needed for multiarch).
+    'AHFAIL_LIBDIR=' + get_option('libdir') + ' cargo build --release -p ahfail-pam --target-dir "@OUTDIR@/target-pam" && cp "@OUTDIR@/target-pam/release/libahfail_pam.so" "@OUTPUT@"'
   ],
   build_by_default: true,
   install: true,