Add SRAM command injection (#17459)

Marlin/src/gcode/queue.h

@@ -66,11 +66,30 @@ public:
   static void clear();
 
   /**
-   * Enqueue one or many commands to run from program memory.
-   * Aborts the current queue, if any.
-   * Note: process_injected_command() will process them.
+   * Next Injected Command (PROGMEM) pointer. (nullptr == empty)
+   * Internal commands are enqueued ahead of serial / SD commands.
    */
-  static void inject_P(PGM_P const pgcode);
+  static PGM_P injected_commands_P;
+
+  /**
+   * Injected Commands (SRAM)
+   */
+  static char injected_commands[64];
+
+  /**
+   * Enqueue command(s) to run from PROGMEM. Drained by process_injected_command_P().
+   * Don't inject comments or use leading spaces!
+   * Aborts the current PROGMEM queue so only use for one or two commands.
+   */
+  static inline void inject_P(PGM_P const pgcode) { injected_commands_P = pgcode; }
+
+  /**
+   * Enqueue command(s) to run from SRAM. Drained by process_injected_command().
+   * Aborts the current SRAM queue so only use for one or two commands.
+   */
+  static inline void inject(char * const gcode) {
+    strncpy(injected_commands, gcode, sizeof(injected_commands) - 1);
+  }
 
   /**
    * Enqueue and return only when commands are actually enqueued
@@ -145,7 +164,10 @@ private:
     #endif
   );
 
-  // Process the next "immediate" command
+  // Process the next "immediate" command (PROGMEM)
+  static bool process_injected_command_P();
+
+  // Process the next "immediate" command (SRAM)
   static bool process_injected_command();
 
   /**