docs: add animated GIF demo for -a flag

Records lolbat -a with vhs, showing live color cycling. Adds Animate
section to demo.md between freq comparison and test suite.

Author: jamesfishwick

demo-animate.tape

@@ -0,0 +1,14 @@
+Output docs/images/demo-animate.gif
+
+Set Theme "Dracula"
+Set FontSize 14
+Set Width 700
+Set Height 300
+Set Framerate 15
+Set PlaybackSpeed 1.0
+
+Type "printf 'Painful zombies quickly watch a jinxed graveyard.\nThe five boxing wizards jump quickly.\nHow vexingly quick daft zebras jump.\nPack my box with five dozen liquor jugs.\nThe quick brown fox jumps over the lazy dog.\n' | ./lolbat -a"
+Enter
+Sleep 4s
+Ctrl+C
+Sleep 500ms

demo.md

@@ -105,6 +105,16 @@ printf "..." | lolbat -mode sine -freq 1.0 -spread 2.0
 
 ![Freq comparison: -mode sine -freq 0.1 / 0.3 / 0.6 / 1.0](docs/images/demo-freq.svg)
 
+## Animate mode
+
+`-a` loops the output continuously, cycling colors on each pass. Interrupt with Ctrl-C.
+
+```bash
+printf "Painful zombies quickly watch a jinxed graveyard.\n..." | lolbat -a
+```
+
+![Animate mode: -a flag](docs/images/demo-animate.gif)
+
 ## Test suite
 
 ```bash