random float

SHADER_CONTRACT.md

@@ -76,7 +76,7 @@ Important rules:
 - Keep parameter IDs, texture IDs, font IDs, and function entry points as valid shader identifiers: letters, numbers, and underscores only, starting with a letter or underscore.
 - Add only controls that are actually used by the shader.
 - Prefer a small number of clear controls with conservative defaults.
-- Keep shaders deterministic unless randomness is an explicit feature. For stable pseudo-randomness, hash from `uv`, pixel coordinates, `frameCount`, or trigger values rather than using unavailable global state.
+- Keep shaders deterministic unless randomness is an explicit feature. For stable process-level variation, use `context.startupRandom`; for per-pixel pseudo-randomness, hash from `uv`, pixel coordinates, `frameCount`, or trigger values.
 - If adapting third-party code, include attribution and source URL in the manifest description when the license allows adaptation.
 - If the source license is unclear or incompatible, do not add the shader package.
 
@@ -149,6 +149,7 @@ struct ShaderContext
     float time;
     float utcTimeSeconds;
     float utcOffsetSeconds;
+    float startupRandom;
     float frameCount;
     float mixAmount;
     float bypass;
@@ -166,6 +167,7 @@ Fields:
 - `time`: elapsed runtime time in seconds.
 - `utcTimeSeconds`: current UTC time of day from the host PC clock, expressed as seconds since UTC midnight.
 - `utcOffsetSeconds`: host PC local UTC offset in seconds. Add this to `utcTimeSeconds` and wrap to `0..86400` to get local time of day.
+- `startupRandom`: random `0..1` value generated once when the host process starts. It stays constant for the lifetime of the app and changes on the next launch.
 - `frameCount`: incrementing frame counter.
 - `mixAmount`: runtime mix amount.
 - `bypass`: `1.0` when the layer is bypassed, otherwise `0.0`.