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-movepane-drag-noflash.sh

@@ -0,0 +1,64 @@
+#!/bin/sh
+
+# Regression test: Alt-dragging a floating pane by its body (move-pane -M,
+# bound by default to M-MouseDrag1Pane/M-MouseDrag1Border) must not
+# retransmit images in other panes of the same window.
+#
+# cmd_join_pane_mouse_move() (cmd-join-pane.c) is a second, separate
+# implementation of "drag to move a floating pane" - parallel to the one in
+# cmd-resize-pane.c already fixed for plain (non-Alt) border drags - and
+# used to call server_redraw_window() unconditionally on every motion
+# event, wiping and retransmitting every image in the window on each step
+# of the drag even though only the floating pane's own rectangle actually
+# moved. See tmux-image-redraw-known-bugs.md for the full write-up.
+#
+# This is checked by counting DCS (\033P) sequences in the client's raw
+# output while a stationary tiled pane's image is present and an unrelated
+# floating pane is Alt-dragged elsewhere in the window: with the fix, none
+# should appear.
+
+. ./image-noflash-common.inc
+
+$TMUX new-session -d -s inner -x 60 -y 20 "printf '$SIXEL_HEADER'; exec sh" ||
+	exit 1
+$TMUX set -g mouse on || fail "set mouse failed"
+FLOAT=$($TMUX new-pane -d -PF '#{pane_id}' -x 16 -y 5 -X 30 -Y 5) ||
+	fail "new-pane -X -Y failed"
+sleep 0.3
+
+[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
+$TMUX set -as terminal-features ',*:sixel' || exit 1
+
+start_capture 60 20
+assert_image_reached
+: >$TMP
+
+FTOP=$($TMUX display-message -p -t "$FLOAT" '#{pane_top}')
+FLEFT=$($TMUX display-message -p -t "$FLOAT" '#{pane_left}')
+GRABCOL=$((FLEFT + 3))
+GRABROW=$((FTOP + 2))
+
+# Alt-drag (Cb meta bit 8, drag bit 32 added while moving) the floating
+# pane by a point inside its body, a few steps in a row.
+seq=$(printf '\033[<8;%s;%sM' "$GRABCOL" "$GRABROW")
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 0.2
+row=$GRABROW
+i=0
+while [ $i -lt 4 ]; do
+	row=$((row + 1))
+	seq=$(printf '\033[<40;%s;%sM' "$GRABCOL" "$row")
+	$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+	sleep 0.15
+	i=$((i + 1))
+done
+seq=$(printf '\033[<8;%s;%sm' "$GRABCOL" "$row")
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 1
+
+NEWTOP=$($TMUX display-message -p -t "$FLOAT" '#{pane_top}')
+[ "$NEWTOP" != "$FTOP" ] || fail "sanity: floating pane did not move (still at $FTOP)"
+
+assert_no_retransmit "while Alt-dragging an unrelated floating pane"
+
+exit 0