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>

README.md

@@ -66,12 +66,14 @@ truth; if anything here disagrees with it, the datasheet wins.
 ## Layout
 
 ```
-platformio.ini        build config (esp32dev, Arduino, Adafruit GFX)
+platformio.ini        build config: envs still_alive (default) and esp32dev
 include/pins.h        pin map
-src/main.cpp          test sketch: cycles through test images
+src/still_alive/      Portal-style end-credits demo (env still_alive)
+src/test_images/      test sketch: cycles through 12 test images (env esp32dev)
 lib/EL512/            local library
   src/EL512Panel.*    framebuffer, row encoder, I2S DMA refresh
   src/EL512_GFX.*     Adafruit_GFX subclass
+tools/                import_still_alive.py
 test/                 unit tests (placeholder)
 docs/                 datasheet and notes
 ```
@@ -79,19 +81,29 @@ docs/                 datasheet and notes
 ## Build and flash
 
 Requires [PlatformIO](https://platformio.org/) (CLI or the VS Code extension).
+The upload and monitor port is set to COM20 in `platformio.ini`.
 
 ```bash
-pio run
-```
-
-```bash
-pio run -t upload
-```
-
-```bash
+pio run -e still_alive -t upload   # end-credits demo
+pio run -e esp32dev -t upload      # test images
 pio device monitor
 ```
 
+## Still Alive demo
+
+A recreation of Portal's ending: timed song text typed into a console on the
+left, credits scrolling top right, ASCII art bottom right. Without extra data it
+plays placeholder text. To import the original text, timing, art and credits from
+[SDSkyKlouD/still-alive-web](https://github.com/SDSkyKlouD/still-alive-web):
+
+```bash
+python tools/import_still_alive.py
+pio run -e still_alive -t upload
+```
+
+This writes `src/still_alive/imported.h`, which is git-ignored. Reset the board and
+start the song when the serial log says (7.75 s after reset with the imported data).
+
 ## Using the driver
 
 ```cpp