Document diagnostic system

Author: JosiahWI

include/tscore/Diags.h

@@ -61,12 +61,26 @@ class DiagsPtr
 {
 public:
   friend Diags *diags();
-  static void   set(Diags *new_ptr);
+
+  /// @brief Registers a Diags instance as the global singleton.
+  ///
+  /// @param new_ptr The Diags instance to register. Must not be nullptr.
+  /// @pre Called during single-threaded process initialization only.
+  /// @post diags() returns new_ptr. The instance is also registered with the
+  ///       DebugInterface subsystem.
+  /// @note Not thread-safe. Must be called exactly once before any logging occurs.
+  static void set(Diags *new_ptr);
 
 private:
   static Diags *_diags_ptr;
 };
 
+/// @brief Returns the global Diags singleton.
+///
+/// @pre DiagsPtr::set() must have been called with a valid Diags instance.
+/// @return Pointer to the global Diags instance, or nullptr if not yet initialized.
+/// @note Thread-safe for reading; the pointer is set once during single-threaded
+///       initialization and never changes thereafter.
 inline Diags *
 diags()
 {