Refactor VM to ExecFrame + tag emitted lines with source script (no behavior change)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine/Vm/ExecFrame.cs

@@ -0,0 +1,15 @@
+using Age.Engine.Model;
+namespace Age.Engine.Vm;
+
+/// <summary>One script activation: the running script, its instruction cursor, its local slots,
+/// its intra-script call/ret stack, and its per-script loop-guard map. Globals live on the VM and
+/// are shared across frames; everything here is per-call and discarded on return.</summary>
+internal sealed class ExecFrame
+{
+    public readonly Script Script;
+    public int Pc;                                       // entry instruction index
+    public readonly Frame Locals = new();
+    public readonly List<int> CallStack = new();         // intra-script `call` (op 0x8f) returns
+    public readonly Dictionary<int, int> EmitSeen = new();
+    public ExecFrame(Script script, int pc) { Script = script; Pc = pc; }
+}