README.adoc: add a nicer demo

I'm not quite sure what to put in it, but it beats a static image.

README.adoc

@@ -14,7 +14,7 @@ MC's Ctrl-o escape, it will always be obvious whether the navigator is running.
 
 'sdn' runs on Linux, all BSD derivatives, and within MSYS2.
 
-image::sdn.png[align="center"]
+image::sdn.webp[align="center"]
 
 Packages
 --------

gen-demo.sh

@@ -0,0 +1,117 @@
+#!/bin/sh -e
+# gen-demo.sh: an automated way of creating an example session recording
+#
+# Prerequisites:
+#  - the parent shell has sdn installed,
+#  - https://git.janouch.name/p/liberty tools/wdye is in PATH.
+
+checkout=/tmp/sdn
+if [ -d "$checkout/.git" ]
+then
+	git -C "$checkout" reset --hard
+	git -C "$checkout" clean -fdx
+else
+	git clone https://git.janouch.name/p/sdn.git "$checkout"
+fi
+
+WDYE_LOGGING=1 wdye /dev/stdin <<'LUA'
+for k, v in pairs(wdye) do _G[k] = v end
+
+local checkout = "/tmp/sdn"
+local shell = spawn {
+	os.getenv "SHELL", "-c", 'cd -- "$1" && exec "$SHELL"',
+	"gen-demo.sh", checkout,
+	environ = {TERM="xterm-256color", SHLVL=false},
+}
+assert(shell, "failed to spawn the user's shell")
+
+local function pause(seconds)
+	expect(timeout {seconds},
+		shell:eof {function () error "shell exited prematurely" end})
+end
+
+local function key(sequence)
+	assert(sequence, "terminal does not define a required key")
+	shell:send(sequence)
+	pause(.75)
+end
+
+local function tap(sequence)
+	assert(sequence, "terminal does not define a required key")
+	shell:send(sequence)
+	pause(.375)
+end
+
+local function type(text)
+	for i = 1, #text do
+		shell:send(text:sub(i, i))
+		if i < #text then pause(.125) end
+	end
+	pause(.75)
+end
+
+local enter = "\r"
+local meta_o = "\27o"
+local meta_t = "\27t"
+
+-- Let the shell finish drawing its initial prompt, then open sdn.
+pause(.5)
+key(meta_o)
+
+-- Toggle hidden entries, unnest one level, calculate directory sizes, nest.
+key "."
+key "."
+key "]"
+key "\0"
+pause(.25)
+key "["
+
+-- Find NEWS through sdn, create ChangeLog -> NEWS, then show the new symlink.
+key(meta_o)
+type "ln -s "
+key(meta_o)
+key "s"
+type "NE"
+key(enter)
+type "ChangeLog"
+key(enter)
+key(meta_o)
+
+-- Exercise both viewers and the editor on a file near the bottom.
+tap "G"
+tap(shell.term.key_up)
+key(shell.term.key_up)
+key(shell.term.key_f3)
+key "q"
+key(shell.term.key_f15)
+key "q"
+key(shell.term.key_f4)
+type ":q"
+key(enter)
+
+-- Select all C++ sources in sdn and insert them into a shell command.
+key(meta_o)
+type "wc -l "
+key(meta_o)
+key "+"
+type "*.cpp"
+key(enter)
+key "t"
+key(enter)
+pause(.5)
+key(meta_o)
+
+-- Return to sdn, cycle through every sort column, and toggle the full view.
+for _ = 1, 6 do tap ">" end
+pause(.25)
+key(meta_t)
+key(meta_t)
+
+key(meta_o)
+shell:send "\4"
+expect(shell:eof {true},
+	timeout {5, function () error "shell did not exit after C-d" end})
+
+local status = shell:wait()
+assert(status == 0, "shell exited abnormally: " .. status)
+LUA