fix: eliminate unsafe code in packet construction and stats counters

Replace all UnsafeCell-based counters with safe alternatives:

- New ShardedCounter: per-thread TLS accumulation (thread_local crate)
  with periodic publish to static AtomicU64. Zero atomic RMW on hot path.
- ACL RuleStatsTracker: remove unsafe Arc<RuleStats> raw pointer
  mutation, use two ShardedCounter fields.
- ZCPacket: replace set_len on uninitialized BytesMut with
  write_bytes + copy_nonoverlapping before set_len.
- StatsManager UnsafeCounter/MetricData: remove UnsafeCell and
  unsafe impl Send/Sync, wrap ShardedCounter. last_updated uses
  AtomicU64 epoch millis.
- Throughput: replace UnsafeCell with AtomicU64.
- secure_datagram: fix grace-window test timestamp.

.rust-analyzer.toml

@@ -0,0 +1,32 @@
+# rust-analyzer config
+# Skip the `easytier-gui` (Tauri) crate because its build scripts require
+# system libraries webkit2gtk-4.1 / javascriptcoregtk-4.1 that are not
+# installed on this host. Excluding it keeps rust-analyzer healthy for the
+# rest of the workspace.
+
+# Override the command used to run build scripts / collect build data.
+[cargo.buildScripts]
+overrideCommand = [
+    "cargo",
+    "check",
+    "--quiet",
+    "--workspace",
+    "--exclude",
+    "easytier-gui",
+    "--message-format=json",
+    "--all-targets",
+    "--keep-going",
+]
+
+# Override the command used for diagnostics-on-save.
+[check]
+overrideCommand = [
+    "cargo",
+    "check",
+    "--workspace",
+    "--exclude",
+    "easytier-gui",
+    "--message-format=json",
+    "--all-targets",
+    "--keep-going",
+]