fix(tools/tmux): unique per-invocation paste buffer; track auto-submit-drafts.sh

Upstream the 2026-07-09 live hot-fix from web1: send-message.sh used a fixed
tmux buffer name (__mosaic_send) shared by all concurrent senders on one
server — load-buffer overwrote load-buffer and paste-buffer -d deleted
underneath peers, cross-delivering or dropping messages. Bit the fleet during
the simultaneous restart (standing briefs swapped between sessions). Buffer
name is now unique per invocation (PID + nanoseconds).

Also brings auto-submit-drafts.sh (coordinator draft-flush watchdog, until now
only a loose file in the deployed ~/.config/mosaic/tools/tmux/) under source
control, and extends test-send-message-socket.sh with a concurrency lock:
5 parallel sends to distinct panes, asserting no drop and no cross-delivery.
The lock fails 3/3 runs against the pre-fix script and passes post-fix.

Without this, the next framework redeploy would regress the live fix.

packages/mosaic/framework/tools/tmux/send-message.sh

@@ -77,10 +77,20 @@ snippet=$(printf '%s' "$MSG" | tr '\n' ' ' | tr -s ' ' | sed 's/[^[:print:]]//g'
 
 # 1) Paste the body as a bracketed paste so multi-line content does not submit
 #    line-by-line. load-buffer/paste-buffer is far safer than `send-keys -l`.
-printf '%s' "$MSG" | "${tmux_cmd[@]}" load-buffer -b __mosaic_send -
+# Buffer name MUST be unique per invocation: concurrent senders on the shared
+# tmux server race a fixed name (load overwrites load, -d deletes underneath),
+# cross-delivering or dropping messages — bit the fleet on the 2026-07-09
+# simultaneous restart (briefs swapped between sessions).
+BUF="__mosaic_send_$$_$(date +%s%N)"
+printf '%s' "$MSG" | "${tmux_cmd[@]}" load-buffer -b "$BUF" -
 # -p = bracketed paste when the client supports it; fall back if not.
-"${tmux_cmd[@]}" paste-buffer -d -p -b __mosaic_send -t "$EFFECTIVE_TARGET" 2>/dev/null \
-  || "${tmux_cmd[@]}" paste-buffer -d -b __mosaic_send -t "$EFFECTIVE_TARGET"
+"${tmux_cmd[@]}" paste-buffer -d -p -b "$BUF" -t "$EFFECTIVE_TARGET" 2>/dev/null \
+  || "${tmux_cmd[@]}" paste-buffer -d -b "$BUF" -t "$EFFECTIVE_TARGET" \
+  || "${tmux_cmd[@]}" delete-buffer -b "$BUF" 2>/dev/null
+# ^ -d deletes the buffer only on a SUCCESSFUL paste; if both attempts fail
+#   (e.g. the target vanished since the liveness check), delete explicitly —
+#   named buffers are exempt from tmux's buffer-limit eviction, so orphans
+#   would otherwise accumulate forever.
 sleep 0.5
 
 # 2) Submit, then verify; flush with another Enter if it is still a draft.