♻️ Drop assert imports from doc fences

`assert` is the sole injected exception: the generated test always binds
it, so a fence must not import it. Revert the README imports and the
generator's node:assert merging; the fences keep the assigned variables
and `assert.equal` calls, and the generator again rejects a fence that
imports node:assert.

scripts/create-doc-tests.ts

@@ -46,14 +46,18 @@ const OUTPUT_DIR = "src/doc-test/__generated__";
 const TYPESCRIPT_LANGS: ReadonlySet<string> = new Set(["ts", "typescript"]);
 
 /**
- * Modules the generated file already binds. An example that imports `node:test`
- * would collide with the prelude `test` binding, so it is surfaced as a fatal
- * error. `node:assert` is allowed: it is merged into the prelude assert import.
+ * Modules the generated file already imports. An example that imports one of
+ * these would collide with the prelude binding (duplicate `test` / `assert`), so
+ * it is surfaced as a fatal error and the example is rewritten.
  */
-const PRELUDE_MODULES: ReadonlySet<string> = new Set(["node:test"]);
+const PRELUDE_MODULES: ReadonlySet<string> = new Set([
+    "node:assert",
+    "node:test",
+]);
 
 /** One line of the prelude every generated file starts with. */
 const PRELUDE_TEST = 'import { test } from "node:test";';
+const PRELUDE_ASSERT = 'import { strict as assert } from "node:assert";';
 
 /** Indentation applied to every fence body line inside the `test` callback. */
 const INDENT = "    ";
@@ -321,17 +325,6 @@ const handleNode = (state: BuilderState, name: string, node: Block): void => {
     }
 };
 
-/**
- * Guarantee `assert` is in scope in every generated file. The import is merged
- * into any `node:assert` import an example already declares, so an example can
- * stay self-contained without colliding with the harness.
- */
-const ensurePreludeAssert = (named: Map<string, NamedImportGroup>): void => {
-    addNamedImport(named, { source: "node:assert", isType: false }, [
-        "strict as assert",
-    ]);
-};
-
 /** Walk one Markdown file and collect its cases and hoisted imports. */
 const parseDoc = (
     name: string,
@@ -346,7 +339,6 @@ const parseDoc = (
     for (const node of fromMarkdown(markdown).children) {
         handleNode(state, name, node);
     }
-    ensurePreludeAssert(state.named);
     return {
         named: state.named,
         passthrough: state.passthrough,
@@ -376,6 +368,7 @@ const buildHeader = (name: string, imports: readonly string[]): string[] => {
         `// Source: ${name}`,
         EMPTY,
         PRELUDE_TEST,
+        PRELUDE_ASSERT,
     ];
     if (imports.length > INITIAL_COUNT) {
         header.push(EMPTY, ...imports);