feat: party log records a group-readable transcript

tmux pipe-pane on every pane of the party session, appending into
<party dir>/log (0640, party group) so any attendee can grab a copy.
Start and stop are announced to everyone attached; close rescues a
non-empty transcript to the host's home before removing the dir.
Panes opened later are picked up by re-running party log (a
pane_pipe check keeps already-piped panes single-piped, since tmux's
pipe-pane -o toggles an existing pipe closed rather than skipping it).
Raw output, escapes included. Close polls the transcript for size
stability before rescuing it, since a cross-filesystem mv is
copy+unlink and could otherwise drop bytes still draining from the
pipe-pane writers.

tests/87-log.bats

@@ -0,0 +1,87 @@
+#!/usr/bin/env bats
+#
+# party log: pipe-pane transcript into the per-party dir. Needs a real
+# tmux server (pipe-pane semantics can't be stubbed honestly), so most
+# tests gate on require_party_group like the other e2e suites.
+
+load 'helpers'
+
+setup() {
+  setup_party_sandbox
+  export TMUX_PARTY_GROUP="$(id -gn)"
+}
+
+teardown() { teardown_party_sandbox; }
+
+logfile() { printf '%s/party-%s:%s.d/log\n' "$PARTY_SOCKET_DIR" "$USER" "$1"; }
+sock_of() { printf '%s/party-%s:%s.d/sock\n' "$PARTY_SOCKET_DIR" "$USER" "$1"; }
+
+# Wait up to ~10s for $2 to appear in file $1. Integer sleeps only:
+# fractional sleep is not portable across the matrix.
+poll_for() {
+  for _ in 1 2 3 4 5 6 7 8 9 10; do
+    grep -q "$2" "$1" 2>/dev/null && return 0
+    sleep 1
+  done
+  return 1
+}
+
+@test "log without a hosted party fails" {
+  run "$PARTY_BIN" log
+  [ "$status" -eq 1 ]
+  [[ "$output" == *"do not host"* ]]
+}
+
+@test "log captures pane output; --stop ends capture" {
+  "$PARTY_BIN" host scribe
+  run "$PARTY_BIN" log
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"Logging to"* ]]
+
+  tmux -S "$(sock_of scribe)" send-keys -t scribe 'echo marker-ONE' Enter
+  poll_for "$(logfile scribe)" marker-ONE
+
+  run "$PARTY_BIN" log --stop
+  [ "$status" -eq 0 ]
+  tmux -S "$(sock_of scribe)" send-keys -t scribe 'echo marker-TWO' Enter
+  sleep 2
+  ! grep -q marker-TWO "$(logfile scribe)"
+}
+
+@test "log file is 0640 so group members can copy it" {
+  "$PARTY_BIN" host perms
+  "$PARTY_BIN" log
+  [ -n "$(find "$(logfile perms)" -prune -perm 0640 2>/dev/null)" ]
+}
+
+@test "re-running log attaches panes created after the first run" {
+  "$PARTY_BIN" host panes
+  "$PARTY_BIN" log
+  tmux -S "$(sock_of panes)" split-window -t panes
+  "$PARTY_BIN" log
+  tmux -S "$(sock_of panes)" send-keys -t panes.1 'echo marker-PANE2' Enter
+  poll_for "$(logfile panes)" marker-PANE2
+}
+
+@test "close rescues a non-empty transcript to HOME" {
+  export HOME="$PARTY_TMP/home"
+  mkdir -p "$HOME"
+  "$PARTY_BIN" host rescue
+  "$PARTY_BIN" log
+  tmux -S "$(sock_of rescue)" send-keys -t rescue 'echo marker-KEEP' Enter
+  poll_for "$(logfile rescue)" marker-KEEP
+  run "$PARTY_BIN" close rescue
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"transcript saved to"* ]]
+  saved=$(ls "$HOME"/party-rescue-*.log)
+  grep -q marker-KEEP "$saved"
+}
+
+@test "close without a transcript stays silent about rescue" {
+  export HOME="$PARTY_TMP/home"
+  mkdir -p "$HOME"
+  "$PARTY_BIN" host quiet
+  run "$PARTY_BIN" close quiet
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"transcript"* ]]
+}