Add four Steins;Gate pages; shuffle the page rotation hourly

Divergence meter (Nixie tubes, world lines and clock), a D-mail sent
through the Phone Microwave using the panel's Message text, the Future
Gadget Lab inventory with live sensors, and a world line chart of today's
solar power. Original pixel art in tools/steins_sprites.py; the sprite
tool is shared between themes and Sprite, sprite(), polar() and arc()
move to el512_pages.h. HomeData gains the message and a 24 h solar
history in 15-minute slots.

The rotation now walks all pages in a random order that is reshuffled
every hour (Still Alive stays out, Now playing is skipped when idle).

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

README.md

@@ -84,6 +84,7 @@ components/el512/       shared driver + ESPHome component (one copy of the code)
   still_alive_data.h    show content, generated by tools/import_still_alive.py
   cyberpunk.*           animated Cyberpunk-style ESPHome pages
   konosuba.*            animated KonoSuba pages; konosuba_sprites.h from tools/
+  steins.*              animated Steins;Gate pages; steins_sprites.h from tools/
   el512_pages.h         home data and helpers shared by the themed pages
   el512_display.*       ESPHome display platform `el512`
   __init__.py, display.py   ESPHome config schema
@@ -92,7 +93,7 @@ src/still_alive/        Portal-style end-credits demo (env still_alive)
 src/test_images/        test sketch: cycles through 12 test images (env esp32dev)
 esphome/                el512-panel.yaml (Home Assistant display), el512-smoke.yaml
 home_assistant/packages/  el512_panel.yaml: helper sensors for the ESPHome pages
-tools/                  import_still_alive.py, konosuba_sprites.py
+tools/                  import_still_alive.py, konosuba_sprites.py, steins_sprites.py
 test/                   unit tests (placeholder)
 docs/                   datasheet and notes
 ```
@@ -111,10 +112,11 @@ pio device monitor
 ## ESPHome / Home Assistant
 
 [esphome/el512-panel.yaml](esphome/el512-panel.yaml) turns the panel into a Home
-Assistant display with sixteen pages: clock and weather, forecast, agenda, a sensor
-dashboard with a message line, now playing, Plex, solar, Still Alive, and eight
-animated themed pages ([Cyberpunk](#cyberpunk-pages), [KonoSuba](#konosuba-pages)). Pages
-rotate on an interval; Home Assistant gets a page select, a pause switch, the
+Assistant display with twenty pages: clock and weather, forecast, agenda, a sensor
+dashboard with a message line, now playing, Plex, solar, Still Alive, and twelve
+animated themed pages ([Cyberpunk](#cyberpunk-pages), [KonoSuba](#konosuba-pages),
+[Steins;Gate](#steinsgate-pages)). Pages
+rotate on an interval, in a random order that is reshuffled every hour; Home Assistant gets a page select, a pause switch, the
 interval, a message text, a "Play Still Alive" button, and Display / Low power mode
 switches (ENABLE, _LOWPWR).
 
@@ -249,6 +251,32 @@ from the series is used. KonoSuba (*Kono Subarashii Sekai ni Shukufuku wo!*) is
 Natsume Akatsuki and Kurone Mishima, © its rights holders; this project is not
 affiliated with or endorsed by them.
 
+## Steins;Gate pages
+
+Four animated pages from the Future Gadget Lab, drawn by
+[components/el512/steins.cpp](components/el512/steins.cpp) with Nixie One, Special
+Elite and Noto Sans JP (plus fonts shared with the other themed pages), all in the
+rotation:
+
+- **Divergence Meter**: eight Nixie tubes roll and lock onto a world line
+  (Steins Gate first), then show the time; every change ripples like a Reading
+  Steiner.
+- **D-Mail**: a flip phone types the panel's Message text (36 characters, or a
+  default) as a D-mail, the Phone Microwave (name subject to change) spins a banana and
+  discharges, and the world line shifts. Upa sits on top; Kurisu keeps watch.
+- **Gadget Lab**: the lab inventory, where every gadget is a live sensor (#008, the
+  Phone Microwave, draws the house load), with a detail card and Okabe in his lab coat.
+- **World Lines**: the alpha, beta and Steins Gate attractor fields with today's solar
+  power plotted as the world line, futures branching off "you are here". The history
+  is kept on the device in 15-minute slots (`HomeData::record()`), so it fills during
+  the day and starts over at midnight or after a restart.
+
+The pixel art lives in [tools/steins_sprites.py](tools/steins_sprites.py) (same
+`--preview` option as the KonoSuba sprites).
+
+These pages are fan art: the pixel art and all texts are original, and no material
+from the series is used. Steins;Gate is © MAGES. and its other rights holders; this project is not affiliated with or endorsed by them.
+
 ## Using the driver
 
 ```cpp