chore(config): add graph-gen package to TypeScript path mappings

Add @bibgraph/graph-gen imports to tsconfig.base.json paths configuration.

Enables imports across the monorepo:
import { GraphSpec } from '@bibgraph/graph-gen';

Path mappings point to source files for TypeScript composite project support:
- @bibgraph/graph-gen → packages/graph-gen/src/index.ts
- @bibgraph/graph-gen/* → packages/graph-gen/src/*

Author: Mearman

tsconfig.base.json

@@ -46,6 +46,8 @@
       "@bibgraph/algorithms/*": ["packages/algorithms/src/*"],
       "@bibgraph/client": ["packages/client/src/index.ts"],
       "@bibgraph/client/*": ["packages/client/src/*"],
+      "@bibgraph/graph-gen": ["packages/graph-gen/src/index.ts"],
+      "@bibgraph/graph-gen/*": ["packages/graph-gen/src/*"],
       "@bibgraph/utils": ["packages/utils/src/index.ts"],
       "@bibgraph/utils/*": ["packages/utils/src/*"],
       "@bibgraph/ui": ["packages/ui/src/index.ts"],