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>

context/steamification.md

@@ -30,7 +30,17 @@ pattern). Forked at BT411 `4e72f0c` (2026-07-14).
   `run/run.cmd` working-tree edit discarded (committed version is the documented solo
   launcher), identity pass (README/CLAUDE.md/CMake `project(bt412)`), this topic +
   roadmap authored.
-- Phase 1 (VS2022 v143): not started.
+- **Phase 1 (VS2022 v143) — DONE 2026-07-14.** Generator moved to
+  `Visual Studio 17 2022` (v143, Win32). One real change was forced: the v143 linker
+  fails the image on unresolved externals even under `/FORCE`, so the dead
+  offline-tool ladders in `game/reconstructed/mech3.cpp` are neutralized behind
+  `BT412_OFFLINE_TOOLS` (default off) and the exe now links `/FORCE:MULTIPLE`
+  (UNRESOLVED retired — real unresolved symbols are hard errors now). None of RP412's
+  other v143 source fixes (TIME.h, NETWORK.h, L4DINPUT, CAMMGR) were needed — BT's
+  tree was already v142-clean. Verified: clean build; solo DEV.EGG runs (gait/target/
+  31-subsystem ticks); two-instance loopback MP via btconsole.py — mesh forms
+  (console + 1502↔1602 game ports, "All connections completed!"), mission runs both
+  sides, each world ticks its master + the remote replicant. [T2]
 - Phases 2–7: not started — see the roadmap.
 
 ## The seams (what plugs in where) [T0 unless noted]
@@ -60,6 +70,23 @@ pattern). Forked at BT411 `4e72f0c` (2026-07-14).
 
 ## Known merge risks (from the 3-way BT411 / RP411-baseline / RP412 comparison)
 
+**Quantified 2026-07-14 [T1]** — BT-vs-RP411-baseline divergence measured with
+`diff --strip-trailing-cr` (baseline extracted from RP412 history, `git show
+3b8b729:MUNGA_L4/<f>`); RP412-side change sizes from `git diff 3b8b729..HEAD`:
+
+| File | BT vs baseline | RP412 change | Treatment |
+|------|----------------|--------------|-----------|
+| `L4RIO.h` | identical | 125 lines (RIOBase split) | RP412 verbatim |
+| `L4RIO.cpp` | identical | 0 | nothing to do |
+| `L4CTRL.h`, `L4NET.H`, `L4PLASMA.*`, `L4KEYBD.h` | identical | small | RP412 verbatim |
+| `L4CTRL.cpp` | 29 diff lines | 17 (PadRIO hook) | apply RP412 diff |
+| `L4GREND.cpp` | 6 diff lines | 15 (PlasmaScreen selector) | apply RP412 diff |
+| `L4VIDEO.cpp` | **2725** diff lines (BT render work) | only 14 | apply RP412 diff — small despite BT divergence |
+| `L4VIDEO.h` | 29 diff lines | 7 | apply RP412 diff |
+| `L4NET.CPP` | 23 diff lines (BT_NET_TRACE ×5) | 345 (seam refactor) | **MEDIUM** — apply seam refactor, re-site traces |
+| `L4VB16.cpp` | **514** diff lines (dev-gauge dock) | 350 (split view) | **HIGH** — true hand-merge |
+| `L4VB16.h` | 14 diff lines | 32 | hand-merge with L4VB16.cpp |
+
 - **`L4VB16.cpp` — HIGH.** RP412 rebuilt ~3846–4046 (splitViews/canvas/present); BT
   rebuilt the same window-management region for dev-gauge docking (`BT_DEV_GAUGES`,
   `BT_DEV_GAUGES_WINDOW`, `BT_DEV_GAUGES_DOCK`, `BT_GAUGE_SCALE`, `BT_GAUGE_SEC_ROT`,
@@ -67,8 +94,6 @@ pattern). Forked at BT411 `4e72f0c` (2026-07-14).
 - **`L4NET.CPP` — MEDIUM.** RP412 rerouted all ~22–26 Winsock call sites through
   `NetTransport_Get()`; BT's 5 `BT_NET_TRACE` diagnostic blocks sit exactly on those
   send/receive sites and must be re-sited onto the seam calls.
-- **`L4RIO.*`, `L4CTRL.cpp`, `L4GREND.cpp` — LOW.** BT never customized these
-  (verified: zero BT_ hits); take RP412's versions after the 3-way check.
 - **CMake**: engine sources are explicit per-file lines (no globs) — every ported
   `.cpp` needs its own `add_library` line or it silently doesn't compile. New deps:
   `xinput9_1_0.lib` (PadRIO), C++/WinRT (KeyLight, per-file C++17), Steamworks SDK +