Reverse thrust was dead on the desktop (user report): wire the 0x3F throttle-handle button

LALT -> button 0x3F was bound in both binding layers and PadRIO pushed the RIO
ButtonPressed event correctly, but nothing ever set the mapper's ReverseThrust:

 - the authentic route is BTL4.RES 'L4' streamed record [2] (Button Throttle1
   0x3F -> attr 6 ReverseThrust@0x124), but our streamed BUTTON mappings are
   not consumed at all -- MechControlsMapper::AddOrErase is still the
   unreconstructed Fail stub, so the event never reaches the attribute;
 - the ONLY writer of reverseThrust was the keyboard bridge's
   'key_throttle < 0' test, which is bypassed whenever a RIO is operational
   (the pad RIO always is, so the bridge is OFF on the desktop) AND is
   unreachable regardless, because L4PADRIO clamps the Throttle channel to
   [0,1].  Net: Alt did nothing and the flag was re-zeroed every frame.

FIX: publish the 0x3F hold state from PadRIO::EmitButton -- the one chokepoint
every desktop source funnels through (keyboard, gamepad, DirectInput joystick,
glass-panel clicks via SetScreenButton) -- and apply it in InterpretControls
gated on BTPadRIOActive() so real pod hardware is untouched (there the streamed
mapping owns the flag).  Marked [T3]; delete once streamed button mappings land.
Reverse is a HOLD (manual: hold the red throttle button, release = forward).

Verified with scratchpad/revtest.py (keybd_event injection, BT_KEY_NOFOCUS):
forward = rev=0 dmd +61.501; LALT held = rev=1 dmd -61.501; release edge logged.

KB: pod-hardware.md now records the streamed-button-mapping gap + this seam.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0166KTsC7ADm7VXEi1HF1jNg

scratchpad/revtest.py

@@ -0,0 +1,88 @@
+"""Reverse-thrust verification: hold W to build throttle, then hold LALT and
+confirm the mapper flips to reverse (speedDemand goes negative).
+
+Injects keys with keybd_event into a btl4 launched with BT_KEY_NOFOCUS=1, so it
+does not need window focus.  Kills ONLY the PID it spawned.
+"""
+import ctypes
+import os
+import re
+import subprocess
+import sys
+import time
+
+VK_W = 0x57
+VK_LMENU = 0xA4          # left Alt
+KEYEVENTF_KEYUP = 0x0002
+
+user32 = ctypes.windll.user32
+
+def hold(vk, seconds):
+    user32.keybd_event(vk, 0, 0, 0)
+    time.sleep(seconds)
+
+def release(vk):
+    user32.keybd_event(vk, 0, KEYEVENTF_KEYUP, 0)
+
+CONTENT = r"C:\git\bt411\content"
+EXE = r"C:\git\bt411\build\Release\btl4.exe"
+LOG = r"C:\git\bt411\scratchpad_rev.log"
+
+if os.path.exists(LOG):
+    os.remove(LOG)
+
+env = dict(os.environ)
+env.update({
+    "BT_START_INSIDE": "1",
+    "BT_KEY_NOFOCUS": "1",
+    "BT_MPPR_TRACE": "1",       # [mppr-c] thr= rev= -> dmd=
+    "BT_PAD_LOG": "1",          # [padwrite] reverse button
+    "BT_LOG": LOG,
+})
+proc = subprocess.Popen([EXE, "-egg", "LAST.EGG"], cwd=CONTENT, env=env)
+print("spawned pid", proc.pid)
+
+try:
+    # wait for the mission to be running (the trace only prints in-mission)
+    deadline = time.time() + 180
+    while time.time() < deadline:
+        if os.path.exists(LOG):
+            txt = open(LOG, errors="replace").read()
+            if "[mppr-c]" in txt:
+                break
+        time.sleep(2)
+    else:
+        print("FAIL: never reached the mapper trace")
+        sys.exit(1)
+
+    # 1) throttle up with W for 3s, no Alt -> expect rev=0, dmd > 0
+    hold(VK_W, 6.0)
+    release(VK_W)
+    time.sleep(1.5)
+    mark = len(open(LOG, errors="replace").read())
+
+    # 2) now hold LALT for 4s -> expect rev=1 and dmd < 0
+    hold(VK_LMENU, 5.0)
+    release(VK_LMENU)
+    time.sleep(1.5)
+
+    txt = open(LOG, errors="replace").read()
+    before = txt[:mark]
+    during = txt[mark:]
+
+    def rows(chunk):
+        return re.findall(r"\[mppr-c\] thr=([-\d.]+) rev=(\d+).*?dmd=([-\d.]+)", chunk)
+
+    b = rows(before)
+    d = rows(during)
+    print("\n--- forward phase (last 3 samples):", b[-3:])
+    print("--- alt-held phase (samples):      ", d[:6])
+    print("--- [padwrite] lines:", re.findall(r"\[padwrite\] reverse button.*", txt)[:4])
+
+    fwd_ok = any(int(r[1]) == 0 and float(r[2]) > 0.5 for r in b)
+    rev_ok = any(int(r[1]) == 1 and float(r[2]) < -0.5 for r in d)
+    print("\nforward (rev=0, dmd>0): %s" % ("PASS" if fwd_ok else "FAIL"))
+    print("reverse (rev=1, dmd<0): %s" % ("PASS" if rev_ok else "FAIL"))
+finally:
+    proc.terminate()
+    print("killed", proc.pid)