BT412: fork from BT411 -- Steam + internet-multiplayer line

Identity pass (README/CLAUDE.md/CMake project bt412), the steamification
roadmap (docs/BT412-ROADMAP.md), and the living context topic
(context/steamification.md). Forked at BT411 4e72f0c; origin is
VWE/BT412.git only -- this repo never touches the BT411 remote.

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

context/steamification.md

@@ -0,0 +1,83 @@
+---
+id: steamification
+title: "BT412 Steamification — status, seams, and merge notes"
+status: living
+source_sections: "docs/BT412-ROADMAP.md; RP412 repo (C:\\vwe\\RP412) git history + docs"
+related_topics: [pod-hardware, multiplayer, gauges-hud, build-and-run, wintesla-port]
+key_terms: [egg, RIO, console]
+open_questions:
+  - "BT cockpit button-bank geometry source (no vRIO CockpitLayout exists for BT): l4gauge.cfg / pod photos / Nick"
+  - "Does BT's rebuilt L4D3D still have RP's NULL-texture AddRef crash pattern?"
+  - "Does L4CONTROLS=PAD set primaryControlType=PrimaryRIO so MechRIOMapper engages unchanged?"
+---
+# BT412 Steamification — status, seams, and merge notes
+
+BT412 = BT411 made distributable on Steam and playable over the internet with no pod
+hardware, following the recipe RP412 proved end-to-end (RP411 → RP412). The full plan
+and phase order: `docs/BT412-ROADMAP.md`. RP412 (`C:\vwe\RP412`) is the reference
+implementation — its git history from commit `467934c` onward is the steamification,
+grouped roughly: toolchain → PadRIO input → MFD-split cockpit → front end/LocalConsole
+→ NetTransport/Steam sockets → lobby → bindings/KeyLight → packaging.
+
+**Repo rule [T0]:** `origin` = `VWE/BT412.git` only. This repo must never touch the
+BT411 remote (no cross-pull remote — deliberate difference from RP412's `rp411`
+pattern). Forked at BT411 `4e72f0c` (2026-07-14).
+
+## Status ledger
+
+- **Phase 0 bootstrap — DONE 2026-07-14.** Tree copied (working tree + full history,
+  `build/` excluded), origin re-pointed, fast-forwarded to `4e72f0c`, dev-state
+  `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.
+- Phases 2–7: not started — see the roadmap.
+
+## The seams (what plugs in where) [T0 unless noted]
+
+- **Input**: `LBE4ControlsManager` PUSH model; `L4CTRL.cpp` parses `L4CONTROLS` tokens;
+  RP412 adds token `PAD` → `new PadRIO()` (a `RIOBase` implementation from
+  XInput+keyboard), and the stock RIO mapper runs against it unchanged. BT's mapper
+  selection: `btl4app.cpp MakeViewpointEntity` switch on
+  `controls->primaryControlType` (+0xc98) → `MechRIOMapper` for PrimaryRIO(4).
+- **Plasma**: BT's plasma is identical hardware to RP's (128×32, `L4PLASMA.h`);
+  RP412's `PlasmaScreen` renders the same `Video8BitBuffered` surface into a desktop
+  window, selected in `L4GREND.cpp` on `L4PLASMA=SCREEN`.
+- **Displays**: BT pod = main 800×600 3D view + 640×480 radar + five mono MFDs on one
+  spanned 1280×480 surface (+ plasma). All cockpit surfaces are bit-plane masks over
+  one shared `SVGA16` pixelBuffer (see [[gauges-hud]], [[pod-hardware]]). RP412's
+  `MFDSplitView` (`L4MFDVIEW.*` + `L4VB16.cpp` split mode, `L4MFDSPLIT=1`) composes
+  child panes on a fixed 1920×1080 canvas; 3D presents into the viewscreen pane via
+  `Present(hDestWindowOverride)`.
+- **Networking**: arcade stack unchanged (see [[multiplayer]]); RP412 moved the wire
+  behind `NetTransport` (`L4NETTRANSPORT.*`), default `WinsockNetTransport`, Steam
+  implementation `L4STEAMTRANSPORT.*` (`ISteamNetworkingSockets`, FakeIP + SDR,
+  degrade-to-TCP on any failure). Addresses stay SOCKADDR_IN-shaped so the egg
+  `[pilots] ip[:port]` roster works over both wires.
+- **Session**: console protocol (egg msgID 3, 1040-byte chunks + ACK; RunMission
+  msgID 5 ×2; console must stay connected) — `tools/btconsole.py` is the working
+  feeder and the spec for the in-process LocalConsole marshal (Phase 5).
+
+## Known merge risks (from the 3-way BT411 / RP411-baseline / RP412 comparison)
+
+- **`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`,
+  `BT_ALL` — 15 sites). Hand-merge; write the 3-way diff notes HERE before editing.
+- **`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 +
+  `steam_api.lib` behind `option(BT412_STEAM)`.
+- **No `/Zp1` in BT's build** (RP's engine builds `/Zp1`): RP412's packing-mismatch
+  workarounds (`WINDOWS_IGNORE_PACKING_MISMATCH`, KeyLight default-packing carve-out,
+  `pack(push,8)` around Steam headers) are no-ops here — keep them for verbatim-ness,
+  but eyeball any copied code for /Zp1-layout assumptions.
+
+## Key Relationships
+- Uses: [[pod-hardware]] · [[multiplayer]] · [[gauges-hud]] · [[build-and-run]]
+- Feeds: [[open-questions]] (steamification investigate items)