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-splitwindow-resize-noflash.sh

@@ -0,0 +1,63 @@
+#!/bin/sh
+
+# Regression test: interactively sizing a new floating pane by dragging
+# (new-pane -M, bound by default to C-MouseDrag1Pane/C-MouseDrag1Empty)
+# must not retransmit images in other panes of the same window on ongoing
+# drag steps.
+#
+# cmd_split_window_mouse_resize() (cmd-split-window.c) used to call
+# server_redraw_window() unconditionally on every motion event while
+# sizing the new floating pane, wiping and retransmitting every image in
+# the window on each step even though only the new pane's own rectangle
+# actually changed. See tmux-image-redraw-known-bugs.md for the full
+# write-up.
+#
+# Creating the floating pane itself causes one legitimate, unrelated
+# redraw (pane creation always redraws the session - see
+# server_redraw_session() in cmd-split-window.c), so this checks DCS
+# (\033P) counts only for the *ongoing* drag-motion steps after that
+# initial creation, once c->tty.mouse_drag_update is already bound to the
+# resize callback: with the fix, none should appear there.
+
+. ./image-noflash-common.inc
+
+$TMUX new-session -d -s inner -x 60 -y 20 "printf '$SIXEL_HEADER'; exec sh" ||
+	exit 1
+$TMUX set -g mouse on || fail "set mouse failed"
+sleep 0.3
+
+[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
+$TMUX set -as terminal-features ',*:sixel' || exit 1
+
+start_capture 60 20
+assert_image_reached
+
+# Ctrl-drag within the pane, well clear of the image, to create and begin
+# sizing a new floating pane. This first press+motion pair both creates the
+# pane (one legitimate redraw) and makes the initial resize call.
+seq=$(printf '\033[<16;30;10M')
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 0.2
+seq=$(printf '\033[<48;35;12M')
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 0.3
+
+n=$($TMUX list-panes | wc -l)
+[ "$n" -eq 2 ] || fail "sanity: floating pane was not created (found $n panes)"
+: >$TMP
+
+# Continue the drag - only ongoing resize-motion events from here, which is
+# what the fix scopes.
+seq=$(printf '\033[<48;38;13M')
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 0.15
+seq=$(printf '\033[<48;40;15M')
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 0.15
+seq=$(printf '\033[<16;40;15m')
+$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+sleep 1
+
+assert_no_retransmit "while sizing a new floating pane"
+
+exit 0