A square root is one node everywhere else, so the shader graph gets its own (#92)

`Sqrt` already exists in the behavior graph (`data/Sqrt.ts`) and in particles
(`ParticleSqrt`); the shader was the only graph without it. `Pow` with exponent
0.5 does the same job, so this is convenience, not a missing capability -- a
radial falloff no longer needs a node whose purpose has to be read off its
parameter.

Like `Pow` it cannot go through `unary()`: GLSL leaves `sqrt` undefined below 0,
so the value is held there with `max(value, 0.0)`. That form takes a float
second argument for a `vec3` too, so unlike `pow` the exponent broadcast has no
counterpart here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/shader/nodes.ts

@@ -383,6 +383,21 @@ export const ShaderPow: ShaderNodeDef = {
   },
 };
 
+/** GLSL leaves `sqrt` undefined below 0, so the value is held at 0 there. */
+export const ShaderSqrt: ShaderNodeDef = {
+  type: 'ShaderSqrt',
+  label: 'Sqrt',
+  category: 'shader',
+  description:
+    'The square root: 0.25 becomes 0.5, 4 becomes 2. Below 0 the value counts as 0. Per channel for colours.',
+  inputs: [anyIn('value', 0)],
+  outputs: [anyOut('result')],
+  params: [],
+  emit: ({ value }) => ({
+    result: { code: `sqrt(max(${value!.code}, 0.0))`, type: value!.type },
+  }),
+};
+
 /** Every node a shader graph offers. Registered by `graph/nodes`, offered by no behavior palette. */
 export const SHADER_NODES: readonly ShaderNodeDef[] = [
   ShaderTextureColor,
@@ -407,6 +422,7 @@ export const SHADER_NODES: readonly ShaderNodeDef[] = [
   ShaderAbs,
   ShaderStep,
   ShaderPow,
+  ShaderSqrt,
 ];
 
 /** A GLSL float literal: always with a decimal point, so `1` does not become an int. */