♻️ Move emit-only options from tsconfig.json to tsconfig.build.json

- Build-only options (target, declaration*, sourceMap, outDir) live in the
  build config where they belong
- Drop redundant options (composite: false is the default;
  allowSyntheticDefaultImports is implied by esModuleInterop; resolveJsonModule
  is unused)
- Drop lib override to inherit es2025+ libs from @tsconfig/node26
- Add explicit include to tsconfig.build.json (include is not inherited via
  extends, only exclude was carrying the file selection by accident)

tsconfig.build.json

@@ -2,8 +2,14 @@
     "extends": "./tsconfig.json",
     "compilerOptions": {
         "noEmit": false,
+        "target": "es2024",
+        "declaration": true,
+        "declarationMap": true,
+        "sourceMap": true,
+        "outDir": "dist",
         "rewriteRelativeImportExtensions": true,
         "rootDir": "src"
     },
+    "include": ["src"],
     "exclude": ["src/**/*.test.ts", "src/**/__tests__/**"]
 }