fix(traceback): 补回 RbpFrames 声明（45d9b55 只带上了门槛改动，编译报未定义名称）

dbghelp 门槛已改写为 if RbpFrames <= 1，但 RbpFrames: int = Frames 的声明未随该提交落盘 -> TPV 重编时 NameError: 未定义名称 RbpFrames（traceback.vp:1635）

traceback.vp

@@ -1612,6 +1612,13 @@ def DumpMainThread(IsBreak: int) -> int:
         EmitText("    [diag] walk: primary=rbp_chain\n")
     # ---- Primary path: RBP chain (no dbghelp) ----
     Frames = WalkRbpChain(Rip, Rbp, Base)
+    # RBP-chain yield ALONE decides the fallback gates below. The stack scan must not mask the
+    # (far stronger) dbghelp unwinder by inflating this count first: measured on
+    # Test/LLvmLiteTest, RBP was 0 (the main thread was blocked inside a system DLL), the chain
+    # produced the single RIP frame, the scan added 1 inferred candidate -> Frames became 2 ->
+    # the `Frames <= 1` dbghelp gate below silently skipped StackWalk64, which is the only path
+    # able to unwind through the frameless ntdll/kernel32 frames and print the real .vp stack.
+    RbpFrames: int = Frames
     if DiagEnabled() != 0:
         N = stdio.snprintf(_State.LineBuf, t.CSizeT(TRACEBACK_LINE_CAP),
                            "    [diag] walk: rbp_chain frames=%d\n", Frames)