Implement Phase A: Stop Dying resilience fixes

Implements all 6 critical resilience fixes from phase-a-stop-dying.md:

- Issue 2.9: Migrate std::sync::RwLock → parking_lot::RwLock (7 files)
  Prevents lock poisoning cascade on writer panic

- Issue 2.2: Add install_panic_hook() to log panics via tracing
  Ensures panics are captured in logs/journald before process death

- Issue 3.7: Add ExecStopPost to systemd service
  Cleans up stale FUSE mounts on service stop

- Issue 2.7: Add check_stale_mount() detection on startup
  Auto-cleans leftover mounts from previous crashes

- Issue 2.10: Integrate sd_notify for systemd lifecycle
  Sends READY=1 after mount, STOPPING on shutdown

- Issue 2.1: Add signal handling with spawn_mount
  Catches SIGTERM/SIGINT for clean shutdown instead of instant death

All 7 Phase A tests pass:
- test_poisoned_tree_lock_returns_eio_not_panic
- test_parking_lot_rwlock_survives_panic
- test_panic_hook_logs_to_tracing
- test_systemd_service_has_execstoppost
- test_stale_mount_check_function_exists
- test_sd_notify_ready_sent
- test_sigterm_triggers_shutdown

musicfs/crates/musicfs-core/Cargo.toml

@@ -12,6 +12,7 @@ tokio = { workspace = true, features = ["sync"] }
 tracing.workspace = true
 xxhash-rust.workspace = true
 hex.workspace = true
+parking_lot.workspace = true
 
 [dev-dependencies]
 tempfile.workspace = true

musicfs/crates/musicfs-core/src/lib.rs

@@ -19,6 +19,38 @@ pub fn sanitize_path(path: &Path) -> String {
         path.to_string_lossy().to_string()
     }
 }
+
+/// Install a custom panic hook that logs panics via tracing before the default behavior.
+/// This ensures panics are captured in log files and journald.
+pub fn install_panic_hook() {
+    let default_hook = std::panic::take_hook();
+    std::panic::set_hook(Box::new(move |info| {
+        let thread = std::thread::current();
+        let thread_name = thread.name().unwrap_or("<unnamed>");
+
+        let message = if let Some(s) = info.payload().downcast_ref::<&str>() {
+            (*s).to_string()
+        } else if let Some(s) = info.payload().downcast_ref::<String>() {
+            s.clone()
+        } else {
+            "unknown panic".to_string()
+        };
+
+        let location = info
+            .location()
+            .map(|l| format!("{}:{}:{}", l.file(), l.line(), l.column()))
+            .unwrap_or_else(|| "unknown location".to_string());
+
+        tracing::error!(
+            thread = thread_name,
+            location = %location,
+            "PANIC: {}",
+            message
+        );
+
+        default_hook(info);
+    }));
+}
 pub use credentials::{Credential, CredentialConfig, CredentialError, CredentialStore};
 pub use error::{Error, Result};
 pub use events::{Event, EventBus};

musicfs/crates/musicfs-core/src/metrics.rs

@@ -1,6 +1,6 @@
+use parking_lot::RwLock;
 use std::collections::HashMap;
 use std::sync::atomic::{AtomicU64, Ordering};
-use std::sync::RwLock;
 use std::time::Instant;
 
 #[derive(Default)]
@@ -45,7 +45,7 @@ impl Metrics {
             self.fuse_ops.open.load(Ordering::Relaxed),
         ));
 
-        for (op, histogram) in self.fuse_latency.histograms.read().unwrap().iter() {
+        for (op, histogram) in self.fuse_latency.histograms.read().iter() {
             let quantiles = histogram.quantiles();
             output.push_str(&format!(
                 "# HELP musicfs_fuse_latency_seconds FUSE operation latency\n\
@@ -95,7 +95,7 @@ impl Metrics {
             "# HELP musicfs_origin_health Origin health status (1=healthy, 0=unhealthy)\n\
              # TYPE musicfs_origin_health gauge\n",
         );
-        for (origin_id, healthy) in self.origin_health.status.read().unwrap().iter() {
+        for (origin_id, healthy) in self.origin_health.status.read().iter() {
             output.push_str(&format!(
                 "musicfs_origin_health{{origin=\"{}\"}} {}\n",
                 origin_id,
@@ -203,7 +203,7 @@ pub struct FuseLatencyMetrics {
 
 impl FuseLatencyMetrics {
     pub fn record(&self, op: &str, latency_secs: f64) {
-        let mut histograms = self.histograms.write().unwrap();
+        let mut histograms = self.histograms.write();
         histograms
             .entry(op.to_string())
             .or_default()
@@ -268,7 +268,6 @@ impl OriginHealthMetrics {
     pub fn set_health(&self, origin_id: &str, healthy: bool) {
         self.status
             .write()
-            .unwrap()
             .insert(origin_id.to_string(), healthy);
     }
 }