bench: symbolic leg-zone targeting, gimp-follow/goto-release, legal egg colors

- BT_SELF_DAMAGE_ZONE=leg resolves the mech's actual leg zone via the
  streamed LegDamageZone flags (new Mech__DamageZone::IsLegZone) -- numeric
  zone ids are per-mech-type and a wrong one aims at a vital (killed the
  limp bench's mech instead of gimping it: 2 ticks, death cycle, respawn
  healed, harness spent -- 3 minutes of healthy circling).
- BT_GOTO=gimp: beeline mode that prefers the nearest LIMPING peer (gimp
  half-states 3/4 ride the one-cell in every update record); stands and
  waits while nobody limps.  BT_GOTO_RELEASE=1 hands the mech back to the
  keyboard once the beeline arrives.
- MP.EGG/MP4.EGG/MP4L.EGG: color=Red and color=Blue are PATCH values, not
  colors (RES vehicletable colors: Black/Brown/Crimson/Green/Grey/Tan/
  White) -- the two pilots carrying them rendered as untextured pastel
  mechs.  Boreas Red->Crimson, Caicias Blue->Grey.  MP4L.EGG = the 4-pilot
  bench egg on the open grass map.

4-node limp verification (user-eyeballed, all instances): the gimped mech
step-drags with the authentic limp on every observer -- no skating (#82
holds in the 4-node overlap case).

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

content/MP4.EGG

@@ -187,7 +187,7 @@ role=Role::Default
 dropzone=one
 vehicle=ava1
 vehicleValue=1000
-color=Red
+color=Crimson
 [127.0.0.1:1702]
 hostType=0
 advancedDamage=1
@@ -201,7 +201,7 @@ role=Role::Default
 dropzone=one
 vehicle=bhk1
 vehicleValue=1000
-color=Blue
+color=Grey
 [127.0.0.1:1802]
 hostType=0
 advancedDamage=1