fix: reject empty removal fixture scans

scripts/lib/removal-fixture.ts

@@ -78,6 +78,9 @@ export async function runtimeImportGraph(
   const files = (await filesBelow(root))
     .filter((file) => /\.(?:[cm]?ts|tsx)$/u.test(file))
     .map((file) => path.resolve(file));
+  if (files.length === 0) {
+    throw new Error("removal fixture scanned module universe is empty");
+  }
   const sourceSet = new Set(files);
   const runtimeRoots = runtimeSourceRoots.map((entry) => path.resolve(root, entry));
   const imports = new Map<string, readonly string[]>();
@@ -124,6 +127,9 @@ export async function removeRuntimeDependentTests(
   runtimeSourceRoots: readonly string[],
 ): Promise<number> {
   const graph = await runtimeImportGraph(root, runtimeSourceRoots);
+  if (graph.dependentTests.length === 0) {
+    throw new Error("removal fixture: no runtime-dependent tests discovered");
+  }
   await Promise.all(graph.dependentTests.map((file) => rm(file, { force: true })));
   return graph.dependentTests.length;
 }