set up scaffolding for dbg engine/frontendm sgs

src/dbg_engine/dbg_engine_core.h

@@ -657,6 +657,31 @@ struct D_EvalVizWindowedRowList
   U64 count_before_semantic;
 };
 
+////////////////////////////////
+//~ rjf: Message Types
+
+typedef struct D_Msg D_Msg;
+struct D_Msg
+{
+  D_MsgKind kind;
+  D_Regs *regs;
+};
+
+typedef struct D_MsgNode D_MsgNode;
+struct D_MsgNode
+{
+  D_MsgNode *next;
+  D_Msg v;
+};
+
+typedef struct D_MsgList D_MsgList;
+struct D_MsgList
+{
+  D_MsgNode *first;
+  D_MsgNode *last;
+  U64 count;
+};
+
 ////////////////////////////////
 //~ rjf: Command Specification Types
 
@@ -957,6 +982,10 @@ struct D_State
   D_RegsNode base_regs;
   D_RegsNode *top_regs;
   
+  // rjf: messages
+  Arena *msgs_arena;
+  D_MsgList msgs;
+  
   // rjf: top-level command batch
   Arena *root_cmd_arena;
   D_CmdList root_cmds;
@@ -1529,6 +1558,17 @@ internal void d_errorf(char *fmt, ...);
 __VA_ARGS__                 \
 })
 
+////////////////////////////////
+//~ rjf: Message Functions
+
+internal void d_msg_(D_MsgKind kind, D_Regs *regs);
+#define d_msg(kind, ...) d_msg_((kind),\
+&(D_Regs)\
+{\
+d_regs_lit_init_top\
+__VA_ARGS__\
+})
+
 ////////////////////////////////
 //~ rjf: Main Layer Top-Level Calls