[temp][do not merge] probe: #770 MSVC LTO facts, #771 member DLL link flags, #769 std.compat

[Sunrisepeak]

Temporary probe PR. Every other workflow is removed on this branch, so only probe-770-771 runs. It measures:

• cl.exe /GL / /bigobj object headers, dumpbin on /GL objects, link / lib with and without /LTCG, /LTCG with /DEBUG and incremental linking, a /GL DLL exporting through __declspec(dllexport), a plain-object DLL with a /GL static library, and import std across /GL boundaries (fix: define native MSVC LTO behavior and its interaction with windows_auto_export #770).
• A workspace member's shared library with build.mcpp link_lib on Windows (fix: preserve package link flags on shared-library link units #771).
• /GL with automatic export discovery, today's lto = true on cl.exe (fix: define native MSVC LTO behavior and its interaction with windows_auto_export #770).
• llvm + MSVC STL import std.compat and whether import std alone compiles std.compat (fix: build std.compat with Clang and MSVC STL #769 follow-ups).

Nothing asserts; results are in the job summaries. Will be closed without merging.