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Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges.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/image-edge-test.sh

@@ -0,0 +1,104 @@
+#!/bin/sh
+
+# Compact visual check for image subcell geometry and layering. Run inside a
+# tmux pane on Kitty or a SIXEL terminal; it needs at least 90 columns/24 rows.
+
+if [ -z "$TMUX" ]; then
+	echo "Run this script inside tmux." >&2
+	exit 1
+fi
+
+columns=$(tput cols 2>/dev/null || echo 0)
+lines=$(tput lines 2>/dev/null || echo 0)
+if [ "$columns" -lt 90 ] || [ "$lines" -lt 24 ]; then
+	echo "This test needs at least 90 columns and 24 rows." >&2
+	exit 1
+fi
+
+at()
+{
+	printf '\033[%s;%sH' "$1" "$2"
+}
+
+kitty()
+{
+	printf '\033_G%s\033\\' "$1"
+}
+
+kitty_solid()
+{
+	id=$1
+	rgba=$2
+	payload=$(i=0; while [ "$i" -lt 256 ]; do
+		printf "$rgba"
+		i=$((i + 1))
+	done | base64 | tr -d '\n')
+	kitty "a=t,q=2,f=32,s=16,v=16,i=$id;$payload"
+}
+
+sixel_folder()
+{
+	printf '\033P0;1q"1;1;26;26#0;2;50;50;50#1;2;0;0;0#2;2;49;49;49#3;2;33;33;33#4;2;47;47;47#5;2;44;44;44#6;2;48;48;48#2?O?C!5?S!4?__?_?__$#0?_wW!5KGwo_-#0?~~!8?!10@BB}w$#2!12?a_A?A?A?A??@E-#0?~~!7?KK~~KK!7?~~$#2!9?K!6?K-#0?F^[owowowowowowowowooS^F$#2?G!10?@@!7?G??G-\033\\'
+}
+
+feature_info=$(tmux display-message -p \
+    'term=#{client_termname} features=#{client_termfeatures} cells=#{client_cell_width}x#{client_cell_height}')
+
+printf '\033[2J\033[H\033[?25l'
+at 1 2
+printf 'tmux image edge-case check'
+at 2 2
+printf '%s' "$feature_info"
+
+kitty_solid 201 '\377\000\000\377'
+kitty_solid 202 '\000\377\000\377'
+kitty_solid 203 '\000\000\377\377'
+
+# The 26-pixel folder occupies whole grid cells, but its visible right and
+# bottom edges must remain partial rather than stretching to those boundaries.
+at 4 2
+printf '1. partial SIXEL: folder should be the same size as direct output'
+at 4 72
+sixel_folder
+
+# Chawan emits X/Y for its image padding. The red square must begin slightly
+# below and right of the [img] anchor, never over it.
+at 8 2
+printf '2. Kitty X/Y: RED begins below/right of the [img] anchor'
+at 8 72
+printf '[img]'
+at 9 72
+kitty 'a=p,q=2,C=1,i=201,p=1,x=0,y=0,w=16,h=16,c=3,r=2,X=6,Y=6'
+
+# Positive Kitty z overlays text and other lower z images; negative z is
+# below text. The shared red/blue edge makes any wrong order obvious.
+at 12 2
+printf '3. Kitty z: BLUE covers RED; GREEN stays behind TEXT'
+at 12 72
+printf 'TEXT'
+at 12 72
+kitty 'a=p,q=2,C=1,i=202,p=2,x=0,y=0,w=16,h=16,c=4,r=2,z=-1'
+at 14 72
+kitty 'a=p,q=2,C=1,i=201,p=3,x=0,y=0,w=16,h=16,c=4,r=2,z=1'
+at 14 74
+kitty 'a=p,q=2,C=1,i=203,p=4,x=0,y=0,w=16,h=16,c=4,r=2,z=2'
+
+# SIXEL is temporal: a later image covers text, while later text destroys the
+# affected image cells. Both results should match on Kitty and SIXEL output.
+at 18 2
+printf '4. SIXEL after text: folder covers FOLDER'
+at 18 72
+printf 'FOLDER'
+at 18 72
+sixel_folder
+at 21 2
+printf '5. SIXEL before text: TEXT WINS, no folder remnant'
+at 21 72
+sixel_folder
+at 21 72
+printf 'TEXT WINS'
+
+at 23 2
+printf 'Take one screenshot, then press Enter to return to the shell.'
+printf '\033[?25h'
+IFS= read -r _image_edge_reply