Add treppe-gl shader renderer for treppe2000

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

README.md

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+# treppe-gl
+
+Render Shadertoy style GLSL shaders onto the treppe2000.
+
+The shader is rendered into a 16x1 float framebuffer (x = stair step) and
+sent as RGBW16 UDP frames to the priority port (3124) of `trepped` on
+`treppe.club.berlin.ccc.de`. While it runs it overrides whatever else the
+stairs show. On exit it sends a black frame. The stairs power must be on
+(Bluescreen > Stairs).
+
+    nix-shell --run "python treppe_gl.py shaders/flow.glsl"
+    nix-shell --run "python treppe_gl.py shaders/pulse.glsl --dry-run"
+
+Options: `--host`, `--port`, `--fps`, `--dry-run` (send nothing),
+`--no-window` (headless), `--white {extract,alpha,off}`.
+
+Shaders define `mainImage(out vec4 fragColor, in vec2 fragCoord)` and may
+use `iResolution`, `iTime`, `iTimeDelta` and `iFrame`. The file is reloaded
+when it changes; on a compile error the last working shader keeps running.
+
+White channel: `extract` (default) sets `w = min(r,g,b)` and subtracts it
+from rgb, `alpha` uses the shader's alpha, `off` sends w = 0.
+
+Debug window: top is the shader at full resolution, bottom are the 16 steps
+as sent (step numbers, colour, white bar). Steps 13-15 are greyed out,
+`trepped` only drives 13 channels. The step to channel mapping is done by
+`trepped`, frames are sent in step order.
+
+Tests: `nix-shell --run "python -m pytest tests"`.