feat(bundler): add files option for in-memory bundling (#25852)

## Summary

Add support for in-memory entrypoints and files in `Bun.build` via the
`files` option:

```ts
await Bun.build({
  entrypoints: ["/app/index.ts"],
  files: {
    "/app/index.ts": `
      import { greet } from "./greet.ts";
      console.log(greet("World"));
    `,
    "/app/greet.ts": `
      export function greet(name: string) {
        return "Hello, " + name + "!";
      }
    `,
  },
});
```

### Features

- **Bundle entirely from memory**: No files on disk needed
- **Override files on disk**: In-memory files take priority over disk
files
- **Mix disk and virtual files**: Real files can import virtual files
and vice versa
- **Multiple content types**: Supports `string`, `Blob`, `TypedArray`,
and `ArrayBuffer`

### Use Cases

- Code generation at build time
- Injecting build-time constants
- Testing with mock modules
- Bundling dynamically generated code
- Overriding configuration files for different environments

### Implementation Details

- Added `FileMap` struct in `JSBundler.zig` with `resolve`, `get`,
`contains`, `fromJS`, and `deinit` methods
- Uses `"memory"` namespace to avoid `pathWithPrettyInitialized`
allocation issues during linking phase
- FileMap checks added in:
  - `runResolver` (entry point resolution)
  - `runResolutionForParseTask` (import resolution)
  - `enqueueEntryPoints` (entry point handling)
  - `getCodeForParseTaskWithoutPlugins` (file content reading)
- Root directory defaults to cwd when all entrypoints are in the FileMap
- Added TypeScript types with JSDoc documentation
- Added bundler documentation with examples

## Test plan

- [x] Basic in-memory file bundling
- [x] In-memory files with absolute imports
- [x] In-memory files with relative imports (same dir, subdirs, parent
dirs)
- [x] Nested/chained imports between in-memory files
- [x] TypeScript and JSX support
- [x] Blob, Uint8Array, and ArrayBuffer content types
- [x] Re-exports and default exports
- [x] In-memory file overrides real file on disk
- [x] Real file on disk imports in-memory file via relative path
- [x] Mixed disk and memory files with complex import graphs

Run tests with: `bun bd test test/bundler/bundler_files.test.ts`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>
Co-authored-by: Dylan Conway <dylan.conway567@gmail.com>

docs/bundler/index.mdx

@@ -220,6 +220,78 @@ An array of paths corresponding to the entrypoints of our application. One bundl
   </Tab>
 </Tabs>
 
+### files
+
+A map of file paths to their contents for in-memory bundling. This allows you to bundle virtual files that don't exist on disk, or override the contents of files that do exist. This option is only available in the JavaScript API.
+
+File contents can be provided as a `string`, `Blob`, `TypedArray`, or `ArrayBuffer`.
+
+#### Bundle entirely from memory
+
+You can bundle code without any files on disk by providing all sources via `files`:
+
+```ts title="build.ts" icon="/icons/typescript.svg"
+const result = await Bun.build({
+  entrypoints: ["/app/index.ts"],
+  files: {
+    "/app/index.ts": `
+      import { greet } from "./greet.ts";
+      console.log(greet("World"));
+    `,
+    "/app/greet.ts": `
+      export function greet(name: string) {
+        return "Hello, " + name + "!";
+      }
+    `,
+  },
+});
+
+const output = await result.outputs[0].text();
+console.log(output);
+```
+
+When all entrypoints are in the `files` map, the current working directory is used as the root.
+
+#### Override files on disk
+
+In-memory files take priority over files on disk. This lets you override specific files while keeping the rest of your codebase unchanged:
+
+```ts title="build.ts" icon="/icons/typescript.svg"
+// Assume ./src/config.ts exists on disk with development settings
+await Bun.build({
+  entrypoints: ["./src/index.ts"],
+  files: {
+    // Override config.ts with production values
+    "./src/config.ts": `
+      export const API_URL = "https://api.production.com";
+      export const DEBUG = false;
+    `,
+  },
+  outdir: "./dist",
+});
+```
+
+#### Mix disk and virtual files
+
+Real files on disk can import virtual files, and virtual files can import real files:
+
+```ts title="build.ts" icon="/icons/typescript.svg"
+// ./src/index.ts exists on disk and imports "./generated.ts"
+await Bun.build({
+  entrypoints: ["./src/index.ts"],
+  files: {
+    // Provide a virtual file that index.ts imports
+    "./src/generated.ts": `
+      export const BUILD_ID = "${crypto.randomUUID()}";
+      export const BUILD_TIME = ${Date.now()};
+    `,
+  },
+  outdir: "./dist",
+});
+```
+
+This is useful for code generation, injecting build-time constants, or testing with mock modules.
+
 ### outdir
 
 The directory where output files will be written.