MP: peer shakiness ROOT-CAUSED as single-box packet jitter (rig, not game) + affinity launcher (task #50)

The residual RANDOM peer shakiness on accel/decel was proven (BT_RXJIT record
inter-arrival probe) to be TEST-RIG packet jitter, not the game: two Debug btl4
nodes on one box contend for CPU, so Windows batches their TCP delivery -- records
arrive in bursts (max 56-226ms gaps, burstiness 3-7x) instead of even ~17ms.  The
peer dead-reckons across the gaps then snaps -> random shake.  Pinning the nodes to
disjoint cores restored even ~17ms delivery (burstiness ~1.0) and the shakiness
vanished (user-confirmed 'that was it').  Real pods = dedicated machines, no
contention -> never see it.  So NO un-authentic jitter buffer -- the coupled
body-channel peer (96a896a/f094d78/23f1532) is the authentic + correct finish.

 - mech.cpp: BT_RXJIT record-arrival-jitter probe (env-gated).
 - tools/mp_launch.sh: 2-node launcher that pins nodes to disjoint cores (bakes in
   the fair-delivery condition; documents why).
 - context/multiplayer.md: the finding, so it is not re-litigated.

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

tools/mp_launch.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# mp_launch.sh -- launch a 2-node local multiplayer test (FOGDAY.EGG) with the two
+# game nodes PINNED TO DISJOINT CPU CORES + the console relay.
+#
+# WHY THE AFFINITY MATTERS (task #50, 2026-07-15): two Debug btl4 nodes on one box
+# fight for the same cores, and Windows punishes the contention by BATCHING their
+# TCP packet delivery -- update records that should arrive every ~17ms instead
+# arrive in bursts (measured gaps up to 226ms, burstiness 3-7x, BT_RXJIT).  A peer
+# dead-reckons across those gaps and then snaps when a record finally lands -> the
+# "random" shaky peer motion.  Pinning each node to its own core set restores even
+# ~17ms delivery (burstiness ~1.0) and the shakiness vanishes.  This is a SINGLE-BOX
+# TEST-RIG artifact -- real pods are dedicated machines and never see it; do NOT
+# "fix" it in the game with an interpolation buffer.
+#
+# Usage:  tools/mp_launch.sh [extra env for BOTH nodes...]
+#   e.g.  tools/mp_launch.sh BT_SLIDE=1
+# Run from the repo root; content/ must hold FOGDAY.EGG and the build in build/Debug.
+set -u
+cd "$(dirname "$0")/../content" || exit 1
+EXE=../build/Debug/btl4.exe
+EXTRA="$*"
+
+powershell -NoProfile -Command "Get-Process btl4,python -ErrorAction SilentlyContinue | Stop-Process -Force" 2>/dev/null
+sleep 2
+rm -f fa.log fb.log console.log
+
+env $EXTRA BT_DEV_GAUGES=1 BT_LOG=fb.log BT_START_INSIDE=1 BT_SPAWN_AT="40 -150"  "$EXE" -egg FOGDAY.EGG -net 1601 &
+sleep 2
+env $EXTRA BT_DEV_GAUGES=1 BT_LOG=fa.log BT_START_INSIDE=1 BT_SPAWN_AT="-40 -150" "$EXE" -egg FOGDAY.EGG -net 1501 &
+sleep 3
+
+# Pin the two nodes to disjoint core sets (0x00F = cores 0-3, 0x3C0 = cores 6-9).
+powershell -NoProfile -Command "\$ps=@(Get-Process btl4 | Sort-Object Id); if(\$ps.Count -ge 2){ \$ps[0].ProcessorAffinity=[IntPtr]0x00F; \$ps[1].ProcessorAffinity=[IntPtr]0x3C0; Write-Output ('PINNED '+\$ps[0].Id+'->cores0-3, '+\$ps[1].Id+'->cores6-9') } else { Write-Output ('WARN only '+\$ps.Count+' btl4 up') }"
+
+sleep 6
+python -u ../tools/btconsole.py FOGDAY.EGG 127.0.0.1:1501 127.0.0.1:1601 > console.log 2>&1 &
+echo "console streaming; nodes on 1501/1601. Logs: content/fa.log content/fb.log"
+wait