regress: rationalise image test coverage, add missing image_support guard

- Extract shared setup for the "noflash" retransmission-avoidance tests
  (image-movepane-drag-noflash.sh, image-splitwindow-resize-noflash.sh,
  image-status-pane-click-noflash.sh, image-selection-noflash.sh) into
  a new image-noflash-common.inc, matching the existing input-common.inc
  convention: dual-server cleanup/fail helpers, the shared SIXEL fixture,
  the attach-and-capture sequence, and the final DCS-retransmission
  assertion. Each test keeps its own distinct interaction and failure
  message; only the copy-pasted scaffolding moved.

- Add the missing #{image_support} runtime guard to
  image-border-status-sixel.sh, which cats a real SIXEL fixture and
  specifically guards a sixel-after-pane-creation ordering bug - without
  it, the DCS sequence is compiled out entirely on a non-image build and
  the test's assertions could pass without exercising anything.
  image-border-status-wipe.sh is deliberately left unguarded: it has no
  image/sixel content at all and guards a general border-status caching
  bug unrelated to ENABLE_IMAGES.

- Reviewed the rest of the branch's regress/image-*.sh tests in full
  (not just headers) for redundant coverage; none warranted merging -
  each guards a genuinely distinct fix via a distinct mechanism.

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

regress/image-border-status-sixel.sh

@@ -0,0 +1,80 @@
+#!/bin/sh
+
+# A SIXEL emitted immediately after creating a floating pane must not erase
+# pane-border-status text. Exercise the same queued new; newp; send-keys path
+# used by the reported reproduction, then repeat it with two tiled panes.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Libss$$ -f/dev/null"
+TMUX2="$TEST_TMUX -Libss-outer$$ -f/dev/null"
+FIXTURE=$(pwd)/monkey-2.sixel.txt
+
+cleanup()
+{
+	$TMUX kill-server >/dev/null 2>&1
+	$TMUX2 kill-server >/dev/null 2>&1
+}
+fail()
+{
+	echo "$*" >&2
+	cleanup
+	exit 1
+}
+expect_title()
+{
+	if ! grep -q "$1" "$TMP"; then
+		missing_titles="$missing_titles $2"
+	fi
+}
+
+cleanup
+TMP=$(mktemp)
+missing_titles=
+trap "cleanup; rm -f $TMP" 0 1 15
+
+$TMUX new-session -d -s inner -x 100 -y 30 sh || fail "new session failed"
+[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
+$TMUX set -g status off
+$TMUX set -g pane-border-status top
+$TMUX set -g pane-border-format '#{pane_title}'
+$TMUX set -as terminal-features ',*:sixel'
+BASE=$($TMUX list-panes -F '#{pane_id}' | head -1)
+$TMUX select-pane -t "$BASE" -T TILED0 || fail "set tiled title failed"
+
+$TMUX2 new-session -d -x 100 -y 30 "$TMUX attach -t inner" ||
+	fail "outer session failed"
+sleep 1
+OUTER=$($TMUX2 list-panes -F '#{pane_id}' | head -1)
+$TMUX2 capture-pane -p -t "$OUTER" >$TMP || fail "initial capture failed"
+grep -q TILED0 $TMP || fail "initial tiled title not visible"
+
+# Keep new-pane and send-keys in one command queue. send-keys targets the new
+# active floating pane, exactly as in: new\; newp\; send-keys "cat ..." enter.
+$TMUX new-pane -T FLOAT0 sh \; send-keys "cat '$FIXTURE'" Enter ||
+	fail "queued floating image command failed"
+sleep 1
+$TMUX2 capture-pane -p -t "$OUTER" >$TMP || fail "capture failed"
+expect_title TILED0 tiled-after-sixel
+expect_title FLOAT0 floating-after-sixel
+
+# Add another tiled pane, then create another floating pane and immediately
+# output the same SIXEL. Every tiled and floating border-status must remain.
+$TMUX select-pane -t "$BASE"
+TILED1=$($TMUX split-window -h -d -PF '#{pane_id}' sh) ||
+	fail "split tiled pane failed"
+$TMUX select-pane -t "$TILED1" -T TILED1 || fail "set second tiled title failed"
+$TMUX new-pane -T FLOAT1 sh \; send-keys "cat '$FIXTURE'" Enter ||
+	fail "queued second floating image command failed"
+sleep 1
+$TMUX2 capture-pane -p -t "$OUTER" >$TMP || fail "capture failed"
+expect_title TILED0 first-tiled-after-split-sixel
+expect_title TILED1 second-tiled-after-split-sixel
+expect_title FLOAT1 floating-after-split-sixel
+[ -z "$missing_titles" ] || fail "missing pane-border-status titles:$missing_titles"
+
+exit 0