Shader graph: alpha input on Output, Texture alpha node (#70)

Output gains a number input `alpha`; unconnected it reads the texel's
alpha, like `color` reads the texel. The epilogue premultiplies with the
resolved alpha. New `Texture alpha` node exposes texel.a for maths.

PortDef.inline=false keeps the alpha port without an inline field: every
number port is inline by default, and a typed default of 0 would have made
every shaded sprite invisible. The compiler's texel fallback is typed:
colour ports get rgb, number ports get texel.a.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/core/shader/compile.ts

@@ -8,6 +8,7 @@ import {
   floatLiteral,
   isShaderNodeDef,
   SHADER_OUTPUT,
+  TEXEL_ALPHA,
   TEXEL_COLOR,
   uniformName,
   vec3Literal,
@@ -89,7 +90,10 @@ export function compileShader(graph: Graph): ShaderBuild {
     return outputs;
   };
 
-  /** What feeds an input: the edge's source, the typed-in value, or – with nothing to type – the texel. */
+  /**
+   * What feeds an input: the edge's source, the typed-in value, or – with nothing to type – the
+   * texel: its colour for a colour port, its alpha for a number port.
+   */
   const input = (node: GraphNode, port: PortDef): Expr | null => {
     const edge = edgesInto(graph, node.id, port.key)[0];
     if (edge) {
@@ -103,7 +107,7 @@ export function compileShader(graph: Graph): ShaderBuild {
     if (port.field?.kind === 'color')
       return { code: vec3Literal(colorOf(typed ?? 0xffffff, 0xffffff)), type: 'vec3' };
     if (port.field) return { code: floatLiteral(Number(typed ?? 0)), type: 'float' };
-    return TEXEL_COLOR;
+    return port.type === 'number' ? TEXEL_ALPHA : TEXEL_COLOR;
   };
 
   const conform = (node: GraphNode, port: PortDef, expr: Expr): Expr | null => {
@@ -119,9 +123,10 @@ export function compileShader(graph: Graph): ShaderBuild {
   if (!output) return { compiled: null, problems, reason: 'No Output node' };
   const outputDef = nodeDef(SHADER_OUTPUT)!;
   const color = input(output, outputDef.inputs[0]!);
+  const alpha = input(output, outputDef.inputs[1]!);
 
   const first = graph.nodes.find((n) => n.id in problems);
-  if (first || !color)
+  if (first || !color || !alpha)
     return { compiled: null, problems, reason: first ? problems[first.id]! : 'Cannot compile' };
 
   const uniforms = shaderParamDefs(graph).map((row) => ({ name: row.key, default: row.default }));
@@ -135,7 +140,8 @@ export function compileShader(graph: Graph): ShaderBuild {
     '    vec4 texel = texture(uTexture, vTextureCoord);',
     `    vec3 ${TEXEL_COLOR.code} = texel.a > 0.0 ? texel.rgb / texel.a : texel.rgb;`,
     ...lines.map((line) => `    ${line}`),
-    `    finalColor = vec4(clamp(${color.code}, 0.0, 1.0) * texel.a, texel.a);`,
+    `    float alpha = clamp(${alpha.code}, 0.0, 1.0);`,
+    `    finalColor = vec4(clamp(${color.code}, 0.0, 1.0) * alpha, alpha);`,
     '}',
     '',
   ].join('\n');