handoff: Steam build gate shipped (both halves) + LAN caveat

scratchpad/SESSION_HANDOFF_20260803.md

@@ -175,3 +175,31 @@ verified EXACT both columns (4-node deaths + 2-node cross-player kills,
 matchcheck-reconciled). NEW 717 item: BUILD HANDSHAKE at join/relay (version
 already in matchlog HDR) -- refuse mismatched builds loudly. Instrumentation
 block unchanged (it discriminates skew vs real bug).
+
+### BUILD HANDSHAKE: SHIPPED for Steam (2026-08-04, 22f732f + d050db5)
+The #108 build-gate item above is DONE for the Steam path (game/glass/
+btl4lobby.cpp, ~60 lines, in the 717 cut):
+- Host lobby stamped btl4ver=BT_VERSION_STRING (unique per zip -- btversion
+  is an always-run custom target, re-stamps from git every build; verified
+  build/btversion.h ticked 749->754 across today's commits).
+- Joiner search filters on exact btl4ver match; the no-lobby log names the
+  local build so field reports self-diagnose (#68 lesson). Post-join
+  GetLobbyData verify covers invites/unstamped hosts: loud log + LeaveLobby.
+- HOST HALF (the part that catches OLD exes, i.e. the actual Conn Man case):
+  PublishSelf adds per-member bv=build; the GO mint SKIPS any non-self
+  member with absent/mismatched bv -> no token in btl4map -> the stale
+  client's own pre-existing "the host's map is missing us" bail fires (that
+  code ships in every old build already). Room screen marks such members
+  [WRONG BUILD -- WILL NOT LAUNCH] so the host sees it BEFORE pressing GO.
+- Old-build member experience: sits in room, GO fires, their exe fails the
+  join back to the FE. They get no in-client message (impossible to add
+  text to an exe they already have) -- the host-side room marker + REJECT
+  LobbyLog line are the operator's tools.
+- Verified: link clean vs the documented 20-stub /FORCE baseline
+  (build-and-run.md:28), gate strings confirmed in the deployed exe
+  (ascii + utf16 scans), content/btl4.exe = committed tree (4.11.754).
+  NOT yet field-tested (needs two Steam machines; single-account can't
+  exercise join-vs-host). First 717 Steam night proves it.
+- NOT COVERED: LAN/relay (join.bat / btconsole relay) -- a stale zip can
+  still join a LAN session. Relay-side version check = separate item if
+  wanted (version already rides the matchlog HDR for post-hoc detection).