Workstream A prototype: play without the cockpit

Splits the control surface the game consumes from the RIO board into
RIOBase (8 virtuals + the five analog scalars); the serial RIO is now one
implementation of it. Adds two new ones:

- PadRIO (L4CONTROLS=PAD): in-process RIO speaking the full surface from
  an XInput controller + PC keyboard using vRIO's default profile (left
  stick/WASD = stick, triggers/Q,E = pedals, right stick Y/PgUp,PgDn =
  rate throttle that holds position, A/Space = trigger, B/R = reverse,
  dpad/arrows = hat, Start,Back/F1,F2 = config). Samples in GetNextEvent
  so button latency does not depend on the 15 s menu-time analog cadence;
  hot-plugs pads; L4PADFLIP=XY inverts stick axes; lamp commands land in
  lampState[] for the planned on-screen cockpit panel. The stock
  VTVRIOMapper/lamp/button path runs unchanged.

- PlasmaScreen (L4PLASMA=SCREEN): the 128x32 plasma glass as a desktop
  window in plasma orange (L4PLASMASCALE, default x4), rendering the same
  Video8BitBuffered surface the gauge system always drew; no COM port.

Verified in the sandbox with vRIO off and no serial devices: boots to a
running mission, controller hot-detected, plasma window drawing live game
content (score readout). BUILD.md 4 documents the desktop environ.ini and
bindings; roadmap updated.

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

BUILD.md

@@ -96,6 +96,47 @@ For runtime debugging the v143 build produces full PDBs — run
 `cdb -g -G -lines -y Release rpl4opt.exe ...` from the working directory
 (cdb ships in this machine's Windows Kits).
 
+### Running without the cockpit (Workstream A prototype)
+
+Two new environment options remove the hardware dependency entirely:
+
+- **`L4CONTROLS=PAD;KEYBOARD`** — selects **PadRIO**
+  ([MUNGA_L4/L4PADRIO.cpp](MUNGA_L4/L4PADRIO.cpp)), an in-process RIO that
+  speaks the full RIO control surface (buttons, axes, lamps) from an XInput
+  controller + the PC keyboard. The stock `VTVRIOMapper` path runs unchanged.
+  Hot-plugging works; with no controller it falls back to keyboard only.
+- **`L4PLASMA=SCREEN`** — renders the 128×32 plasma display as its own
+  desktop window ("Plasma Display", plasma orange, `L4PLASMASCALE` sets the
+  pixel size, default 4). No COM port. Closing the window hides it.
+
+Bindings (vRIO's default profile, condensed):
+
+| Input | Pod control |
+|-------|-------------|
+| Left stick / WASD | joystick X/Y |
+| LT / RT, Q / E | left / right pedal |
+| Right stick Y, PgUp / PgDn | throttle (rate; position holds) |
+| A / Space | joystick trigger |
+| B / R | reverse thrust (ButtonThrottle1) |
+| DPad / arrows | joystick hat (look) |
+| X, Y, LB, RB | pinky / thumb-low / thumb-low / thumb-high |
+| Start / F1, Back / F2 | config buttons (AuxUpperRight 1 / 2) |
+
+`L4PADFLIP=XY` (or `X` / `Y`) inverts the stick axes if the feel is wrong.
+Example desktop `environ.ini`:
+
+```ini
+L4CONTROLS=PAD;KEYBOARD
+DPLARG=1
+L4DPLCFG=RPDPL.INI
+L4GAUGE=640x480x16
+L4PLASMA=SCREEN
+TARGETFPS=60
+```
+
+Lamp commands land in `PadRIO::lampState[]` — the hook for the planned
+on-screen cockpit panel (vRIO buttons arranged around the displays).
+
 Also note: `Fail()` compiles to `abort()` in release (`MUNGA/DEBUGOFF.h`) —
 e.g. running with `L4CONTROLS=KEYBOARD` alone aborts at VTV creation because
 no keyboard-only pod mapper exists (the old CRT showed a blocking R6010