feat(port): PE 上的异常机制跟随我们带的 unwinder

clang 面向这个三元组会定义 __SEH__,因为那是 mingw 在这个架构上用的东西 ——
而 libunwind 自己的公开头于是去够 <windows.h> 和 <ntverp.h>:

  unwind.h:22 → …/mingw32/include/windows.h → crtdefs.h
              → corecrt.h:98 typedef redefinition

⚠️ 和这个目标上其它每一条失败同一类:一个平台宏被读成了「底下是什么」的陈述。
这里答案是一个 flag 而不是一份覆盖,因为问题确实是一个构建决定:这个程序的
unwinder 就是本包里的这个,而它读 DWARF 表 —— 和它在 ELF 上、Mach-O 上读的
是同一批。

⚠️ 它必须到达图里每一个翻译单元(docs/13 记着 -fno-exceptions 的同一条理由:
模型记在 BMI 里)。写在这里覆盖本包;消费者也要写一遍,而由 capability 决定
才是能消掉这份重复的形状。

⚠️ 我第一次把它写在了 block header **之前**,于是它落进了上一个块 —— 实测
compile_commands 里根本没有这个 flag,而报错一字未变。

Author: speak-agent

mcpp.toml

@@ -219,7 +219,31 @@ sources = [
   "!llvm/compiler-rt/lib/builtins/*xc3.c",
 ]
 
+# ⭐ THE EXCEPTION MECHANISM FOLLOWS THE UNWINDER WE SHIP, NOT THE PLATFORM'S
+# DEFAULT.
+#
+# clang targeting this triple defines `__SEH__`, because that is what mingw uses
+# on this architecture — and libunwind's own public header then reaches for
+# `<windows.h>` and `<ntverp.h>`:
+#
+#     unwind.h:22 → /usr/x86_64-w64-mingw32/include/windows.h
+#                 → crtdefs.h → corecrt.h:98 typedef redefinition
+#
+# ⚠️ That is the same class of failure as every other one on this target: a
+# vendor SDK reached because a platform macro was read as a statement about what
+# is underneath. Here the answer is a flag rather than an overlay, because the
+# question is genuinely a build decision: this program's unwinder is the one in
+# this package, and it reads DWARF tables — the same ones it reads on ELF and on
+# Mach-O.
+#
+# ⚠️ IT HAS TO REACH EVERY TRANSLATION UNIT IN THE GRAPH, for the reason
+# docs/13 records about `-fno-exceptions`: the model is recorded in a BMI, and a
+# dependency compiled one way cannot be imported by a unit compiled the other.
+# Stated here it covers this package; a consumer states it too, and mcpp
+# deciding it from the capability is the shape that would remove the repetition.
 [target.'cfg(windows)'.build]
+cxxflags = ["-fdwarf-exceptions"]
+cflags   = ["-fdwarf-exceptions"]
 sources = [
   "!llvm/libcxx/src/filesystem/int128_builtins.cpp",
   "llvm/compiler-rt/lib/builtins/*.c",