Implement generalised image support

Support Kitty and SIXEL input and output with grid-resident image layers, history and copy-mode integration, text fallback rendering, and damage-rectangle redraws.

Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges. The original history is preserved in backup/4902-image-support-before-master-rebase-20261003.

tools/sixel-scroll-region-test.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+# Visual test for sixel-region-scrolling: does this terminal actually move
+# SIXEL pixels along with tmux's own scroll-region escapes (DECSTBM/DECSLRM
+# + IND/RI/RIN), or does tmux need to fall back to redrawing the image on
+# every scroll?
+#
+# There is no way to query a terminal for this, so it has to be judged by
+# eye. Run this from inside the tmux session you want to test (attached
+# from whichever terminal emulator you're checking), and watch the
+# numbered colour bands in the floating pane while the counter below it
+# scrolls slowly. Run it once with the option on, once with it off, and
+# compare:
+#
+#   - "off" is the known-good baseline: tmux redraws the whole image every
+#     scroll, so the bands must stay perfectly aligned with the counter no
+#     matter what the terminal does on its own.
+#   - "on" trusts the terminal to have moved the pixels itself. If it
+#     looks identical to "off", this terminal is fine. If a band freezes,
+#     duplicates, tears, or drifts out of sync with the counter, this
+#     terminal does not scroll SIXEL regions correctly and
+#     sixel-region-scrolling should stay off for it.
+#
+# Usage: sh tools/sixel-scroll-region-test.sh [on|off]
+
+set -eu
+cd "$(dirname "$0")/.."
+
+MODE=${1:-on}
+case "$MODE" in
+on|off) ;;
+*) echo "usage: $0 [on|off]" >&2; exit 1 ;;
+esac
+
+tmux set -s sixel-region-scrolling "$MODE"
+tmux set -as terminal-features ',*:sixel'
+
+echo "Testing with sixel-region-scrolling=$MODE. Watch the coloured bands" >&2
+echo "in the floating pane - they should stay locked to the counter." >&2
+sleep 2
+
+PANE=$(tmux new-pane -d -x 24 -y 22 -PF '#{pane_id}' \
+	"cat '$(pwd)/tools/sixel-ruler.six'; for i in \$(seq 1 50); do echo line \$i; sleep 0.3; done; echo done - press enter; read _")
+tmux select-pane -t "$PANE"