Move BuildError to errors.ts

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

tools/build/errors.ts

@@ -0,0 +1,3 @@
+
+// User-facing build failure: printed without a stack trace.
+export class BuildError extends Error {}

tools/build/graph.ts

@@ -1,7 +1,8 @@
 
 import type {RuleDecl} from './api.ts';
+import {BuildError} from './errors.ts';
 
-export class BuildError extends Error {}
+export {BuildError};
 
 export interface GraphResult {
     order : RuleDecl[];           // topological, stable w.r.t. declaration order