Add Portal-style Still Alive demo and split build envs

- src/still_alive: timed console text, typed scrolling credits and ASCII art,
  laid out for 512x256 (41x31 console, 41x9 credits, 41x20 picture).
  Plays placeholder content unless imported.h exists.
- tools/import_still_alive.py: generates the git-ignored imported.h (text,
  timing, art, credits) from SDSkyKlouD/still-alive-web.
- platformio.ini: envs still_alive (default) and esp32dev (test images,
  moved to src/test_images).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

platformio.ini

@@ -1,7 +1,10 @@
 ; Planar EL512.256-H electroluminescent panel driver for ESP32 (ESP-WROOM-32)
 ; https://docs.platformio.org/page/projectconf.html
 
-[env:esp32dev]
+[platformio]
+default_envs = still_alive
+
+[env]
 platform = espressif32@6.10.0
 board = esp32dev
 framework = arduino
@@ -12,3 +15,11 @@ lib_deps =
     adafruit/Adafruit GFX Library
 build_flags =
     -Wall
+
+; Test images: cycles through 12 labelled test patterns.
+[env:esp32dev]
+build_src_filter = +<test_images/>
+
+; Portal-style end-credits console.
+[env:still_alive]
+build_src_filter = +<still_alive/>