Net: the Steam transport -- FakeIP + SDR behind the wire seam (step 4b)

NEW gated (BT_STEAM) TU L4STEAMNET: ISteamNetworkingSockets with FakeIP --
pseudo-SOCKET handles (0x5EA0xxxx) behind the L4NET BTNet* seam; the engine
TCP byte stream rides reliable-NoNagle Steam messages re-assembled into
per-connection rings (nonblocking recv semantics preserved: empty ->
WSAEWOULDBLOCK, peer-closed -> 0); two FakeIP ports mirror the arcade
console/game channel convention; listen sockets + accept queues via the
global status-changed callback; graceful degrade to Winsock at every step.
Seam completions in L4NET.CPP: CheckSocket reports a pseudo-socket peer as
its FAKE ipv4:port (the lobby egg roster matches unchanged); OpenConnection
TCP_OPEN delegates FakeIP targets to BTSteamNet_Connect.  WinMain installs
on BT_STEAM_NET=1 (lobby launch path / by hand).  CMake: gated SDK include/
lib/dll-copy.  Both pumps (SteamAPI_RunCallbacks + sockets RunCallbacks)
drive it -- the FakeIP result rides the GENERAL dispatch (first attempt with
only the sockets pump timed out).

Verified live (ON/ON build, Steam client running): SteamAPI_Init OK,
steam_appid.txt(480) auto-written, FakeIP allocated
(169.254.36.58, console 54464 / game 54465); without BT_STEAM_NET or
without Steam the game stays pure Winsock.

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

content/LAST.EGG

@@ -1,8 +1,8 @@
 [mission]
 adventure=BattleTech
-map=cavern
+map=grass
 scenario=freeforall
-time=night
+time=day
 weather=clear
 temperature=27
 length=600
@@ -156,9 +156,8 @@ x=128
 y=32
 width=8
 [pilots]
-pilot=127.0.0.1:1502
-pilot=127.0.0.1:1602
-[127.0.0.1:1502]
+pilot=200.0.0.96
+[200.0.0.96]
 hostType=0
 advancedDamage=1
 loadzones=1
@@ -172,20 +171,6 @@ dropzone=one
 vehicle=bhk1
 vehicleValue=1000
 color=White
-[127.0.0.1:1602]
-hostType=0
-advancedDamage=1
-loadzones=1
-name=Boreas
-bitmapindex=2
-experience=expert
-badge=VGL
-patch=Yellow
-role=Role::Default
-dropzone=one
-vehicle=ava1
-vehicleValue=1000
-color=Red
 [largebitmap]
 bitmap=BitMap::Large::Aeolus
 [BitMap::Large::Aeolus]
@@ -242,4 +227,3 @@ width=4
 model=dfltrole
 [Role::NoReturn]
 model=noretun
-