feat(bundler): expose reactFastRefresh option in Bun.build API (#25731)

Fixes #25716

Adds support for a `reactFastRefresh: boolean` option in the `Bun.build`
JavaScript API, matching the existing `--react-fast-refresh` CLI flag.

```ts
const result = await Bun.build({
    reactFastRefresh: true,
    entrypoints: ["src/App.tsx"],
});
```

When enabled, the bundler adds React Fast Refresh transform code
(`$RefreshReg$`, `$RefreshSig$`) to the output.

packages/bun-types/bun.d.ts

@@ -1942,6 +1942,16 @@ declare module "bun" {
       development?: boolean;
     };
 
+    /**
+     * Enable React Fast Refresh transform.
+     *
+     * This adds the necessary code transformations for React Fast Refresh (hot module
+     * replacement for React components), but does not emit hot-module code itself.
+     *
+     * @default false
+     */
+    reactFastRefresh?: boolean;
+
     outdir?: string;
   }
 

src/bun.js/api/JSBundler.zig

@@ -7,6 +7,7 @@ pub const JSBundler = struct {
         target: Target = Target.browser,
         entry_points: bun.StringSet = bun.StringSet.init(bun.default_allocator),
         hot: bool = false,
+        react_fast_refresh: bool = false,
         define: bun.StringMap = bun.StringMap.init(bun.default_allocator, false),
         loaders: ?api.LoaderMap = null,
         dir: OwnedString = OwnedString.initEmpty(bun.default_allocator),
@@ -341,6 +342,10 @@ pub const JSBundler = struct {
                 }
             }
 
+            if (try config.getBooleanLoose(globalThis, "reactFastRefresh")) |react_fast_refresh| {
+                this.react_fast_refresh = react_fast_refresh;
+            }
+
             var has_out_dir = false;
             if (try config.getOptional(globalThis, "outdir", ZigString.Slice)) |slice| {
                 defer slice.deinit();

src/bundler/bundle_v2.zig

@@ -1904,6 +1904,7 @@ pub const BundleV2 = struct {
             transpiler.options.css_chunking = config.css_chunking;
             transpiler.options.banner = config.banner.slice();
             transpiler.options.footer = config.footer.slice();
+            transpiler.options.react_fast_refresh = config.react_fast_refresh;
 
             if (transpiler.options.compile) {
                 // Emitting DCE annotations is nonsensical in --compile.

test/regression/issue/25716.test.ts

@@ -0,0 +1,49 @@
+// https://github.com/oven-sh/bun/issues/25716
+// Expose `--react-fast-refresh` option in `Bun.build` JS API
+import { expect, test } from "bun:test";
+import { tempDirWithFiles } from "harness";
+import { join } from "path";
+
+test("Bun.build reactFastRefresh option enables React Fast Refresh transform", async () => {
+  const dir = tempDirWithFiles("react-fast-refresh-test", {
+    "component.tsx": `
+      import { useState } from "react";
+
+      export function Counter() {
+        const [count, setCount] = useState(0);
+        return <button onClick={() => setCount(count + 1)}>{count}</button>;
+      }
+
+      export default function App() {
+        return <div><Counter /></div>;
+      }
+    `,
+  });
+
+  // With reactFastRefresh: true, output should contain $RefreshReg$ and $RefreshSig$
+  const buildEnabled = await Bun.build({
+    entrypoints: [join(dir, "component.tsx")],
+    reactFastRefresh: true,
+    target: "browser",
+    external: ["react"],
+  });
+
+  expect(buildEnabled.success).toBe(true);
+  expect(buildEnabled.outputs).toHaveLength(1);
+
+  const outputEnabled = await buildEnabled.outputs[0].text();
+  expect(outputEnabled).toContain("$RefreshReg$");
+  expect(outputEnabled).toContain("$RefreshSig$");
+
+  // Without reactFastRefresh (default), output should NOT contain refresh calls
+  const buildDisabled = await Bun.build({
+    entrypoints: [join(dir, "component.tsx")],
+    target: "browser",
+    external: ["react"],
+  });
+
+  expect(buildDisabled.success).toBe(true);
+  const outputDisabled = await buildDisabled.outputs[0].text();
+  expect(outputDisabled).not.toContain("$RefreshReg$");
+  expect(outputDisabled).not.toContain("$RefreshSig$");
+});