Toolchain: VS2022 (v143) is the build of record -- /FORCE:UNRESOLVED retired

The v143 linker refuses to emit an image with unresolved externals even
under /FORCE, so the dead offline-tool ladders in mech3.cpp
(Mech::CreateSubsystemStream / SubsystemDefaultData -- never called at
runtime, resources ship prebuilt in BTL4.RES) are compiled out behind
BT412_OFFLINE_TOOLS (default off). The exe links /FORCE:MULTIPLE only:
a genuinely unresolved symbol is now a hard link error instead of a
hidden runtime AV. No other source changes needed for v143.

Verified: clean build; solo DEV.EGG runs; two-instance loopback MP via
btconsole.py -- mesh forms, mission runs both sides, master + replicant
tick in each world. (Phase 1 of docs/BT412-ROADMAP.md)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -322,10 +322,12 @@ target_link_libraries(btl4 PRIVATE
     "${DXSDK}/Lib/x86/dxguid.lib"
     "${DXSDK}/Lib/x86/DxErr.lib"
     winmm dbghelp shell32 user32 gdi32 ole32 ws2_32)
-# /FORCE: the 1995 headers define free funcs/globals without inline/extern, so
-# identical symbols land in every TU (~124 LNK2005); MULTIPLE keeps the first.
-# UNRESOLVED tolerates the dead offline-tool factory in mech3.cpp.  See docs.
-target_link_options(btl4 PRIVATE /FORCE)
+# /FORCE:MULTIPLE: the 1995 headers define free funcs/globals without
+# inline/extern, so identical symbols land in every TU (~124 LNK2005);
+# MULTIPLE keeps the first.  UNRESOLVED is gone (BT412/v143): the dead
+# offline-tool factory in mech3.cpp is neutralized behind BT412_OFFLINE_TOOLS,
+# so any unresolved external is now a REAL link error, not a runtime AV.
+target_link_options(btl4 PRIVATE /FORCE:MULTIPLE)
 
 # Copy the third-party runtime DLLs next to the built exe.
 add_custom_command(TARGET btl4 POST_BUILD