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/Cargo.lock

@@ -1911,6 +1911,7 @@ dependencies = [
  "musicfs-core",
  "musicfs-origins",
  "musicfs-sync",
+ "parking_lot 0.12.5",
  "rmp-serde",
  "serde",
  "sled",
@@ -1934,6 +1935,8 @@ dependencies = [
  "musicfs-fuse",
  "musicfs-metadata",
  "musicfs-origins",
+ "parking_lot 0.12.5",
+ "sd-notify",
  "tokio",
  "tracing",
  "tracing-appender",
@@ -1946,6 +1949,7 @@ name = "musicfs-core"
 version = "0.1.0"
 dependencies = [
  "hex",
+ "parking_lot 0.12.5",
  "serde",
  "serde_json",
  "tempfile",
@@ -2015,6 +2019,7 @@ dependencies = [
  "dashmap",
  "libc",
  "musicfs-core",
+ "parking_lot 0.12.5",
  "tempfile",
  "thiserror 1.0.69",
  "tokio",
@@ -2080,6 +2085,7 @@ dependencies = [
  "async-trait",
  "bytes",
  "fail",
+ "libc",
  "musicfs-cache",
  "musicfs-cas",
  "musicfs-core",
@@ -2089,6 +2095,7 @@ dependencies = [
  "parking_lot 0.12.5",
  "reqwest",
  "rlimit",
+ "sd-notify",
  "tempfile",
  "thiserror 1.0.69",
  "tokio",
@@ -2939,6 +2946,15 @@ dependencies = [
  "untrusted",
 ]
 
+[[package]]
+name = "sd-notify"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b943eadf71d8b69e661330cb0e2656e31040acf21ee7708e2c238a0ec6af2bf4"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "security-framework"
 version = "3.7.0"