feat(tools): record an editor demo of the header fan-out

Adds a recording harness for the Emacs session alongside the terminal one, and
the capture it produces: a declaration added to rt_scene.h, the fourteen
translation units across four targets that the depfiles put downstream of it,
and the link steps that do not rerun because the objects come back byte
identical.

Both harnesses waited on xdotool search --sync for their window, which blocks
indefinitely rather than failing when the window never appears, so a recording
that went wrong hung instead of exiting. Replaced with a bounded poll.

tools/record-kitty.sh

@@ -26,7 +26,11 @@ kitty \
 KITTY_PID=$!
 sleep 3
 
-xdotool search --sync --onlyvisible --class kitty windowactivate >/dev/null 2>&1 || true
+for _ in $(seq 30); do
+    xdotool search --onlyvisible --class kitty >/dev/null 2>&1 && break
+    sleep 1
+done
+xdotool search --onlyvisible --class kitty windowactivate >/dev/null 2>&1 || true
 xdotool type --delay 10 -- 'export PS1="raytracer \$ " PATH='"$WORK"'/bin:$PATH; clear'
 xdotool key Return
 sleep 1