Walk-up callsign/mech request: join menu -> relay -> the session egg

The 1995 front-desk conversation, over the internet.  join.bat/
join_lan.bat now open a JOIN-GAME menu (the FE in BT_FE_JOIN trim:
CALLSIGN edit + the 18-mech list + JOIN); the choice relaunches into the
normal join with BT_CALLSIGN/BT_MECH env, rides the relay SEAT_REQUEST
as {callsign NUL mech NUL} (empty payload = roster defaults, wire-
compatible), and the relay validates the mech tag, HOLDS egg delivery
until every pod's console pad is connected, rewrites the egg via
eggmodel (vehicle= + rasterized callsign bitmaps, graceful no-PySide6
fallback) and streams it to all pads -- so every player's egg copy
carries every player's callsign.

The hold is gated on CONSOLE-PAD count, not game-side registration: a
pod HELLOs only after parsing the egg's roster, so a registration gate
deadlocks (hit live in the first run; pods animate in WAITING FOR
MISSION ASSIGNMENT during the hold).

Verified 2-node e2e (env-injected requests): thor/vulture requested over
roster defaults bhk1/ava1 -> relay held 1/2, released at 2/2, rewrote
both seats (relay log), the delivered egg carries vehicle=thor/vulture +
name=VIPER/MONGOOSE, both pods launched and built [cyl] tables for both
requested mechs.  Join-menu layout screenshot-verified; the menu's
click-through relaunch awaits live human verification.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

players/join_lan.bat

@@ -16,7 +16,10 @@ set BT_LOG=join.log
 set BT_START_INSIDE=1
 set BT_DEV_GAUGES=1
 cd content
-..\build\Release\btl4.exe -egg OPERATOR.EGG -net 1501
+rem JOIN-GAME menu (2026-07-22): pick your CALLSIGN + MECH, click JOIN --
+rem your choice is sent to the operator and lands in the mission roster.
+set BT_FE_JOIN=1
+..\build\Release\btl4.exe
 echo.
 echo The game has exited.  If you did NOT quit on purpose
 echo (crash, or it never connected), send the operator the