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/graph/NodeRegistry.ts

@@ -14,6 +14,8 @@ export interface PortDef {
   default?: PropertyValue;
   /** The property this port carries; its typed-in value is edited as that property (color, enum, …). */
   field?: PropertyDef;
+  /** `false`: nothing to type in even for a number – unconnected, the node's own rule applies. */
+  inline?: boolean;
 }
 
 /**
@@ -145,6 +147,7 @@ export function inlineInputs(def: NodeDef): PortDef[] {
 }
 
 function isInlinePort(port: PortDef): boolean {
+  if (port.inline === false) return false;
   if (port.type === 'vec3') return port.field !== undefined;
   return port.type !== 'exec' && port.type !== 'object' && port.type !== 'struct';
 }