Document static routes vs file routes in HTTP server (#21506)

## Summary
- Add comprehensive documentation distinguishing static routes from file
routes in `Bun.serve()`
- Document caching behavior differences: ETag vs Last-Modified
- Explain performance characteristics and error handling differences
- Provide clear use case recommendations

## Changes
- **File Responses vs Static Responses** section explaining the two
approaches
- **HTTP Caching Behavior** section detailing ETag and Last-Modified
support
- **Status Code Handling** section covering automatic 204/304 responses
- Code examples showing both patterns with clear explanations

## Key Documentation Points
- Static routes (`new Response(await file.bytes())`) buffer content in
memory at startup
- File routes (`new Response(Bun.file(path))`) read from filesystem per
request
- Static routes use ETags for caching, file routes use Last-Modified
headers
- File routes handle 404s automatically, static routes cause startup
errors for missing files
- Both support HTTP caching standards but with different validation
strategies

🤖 Generated with [Claude Code](https://claude.ai/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>

docs/api/http.md

@@ -164,6 +164,70 @@ Static responses do not allocate additional memory after initialization. You can
 
 Static route responses are cached for the lifetime of the server object. To reload static routes, call `server.reload(options)`.
 
+### File Responses vs Static Responses
+
+When serving files in routes, there are two distinct behaviors depending on whether you buffer the file content or serve it directly:
+
+```ts
+Bun.serve({
+  routes: {
+    // Static route - content is buffered in memory at startup
+    "/logo.png": new Response(await Bun.file("./logo.png").bytes()),
+
+    // File route - content is read from filesystem on each request
+    "/download.zip": new Response(Bun.file("./download.zip")),
+  },
+});
+```
+
+**Static routes** (`new Response(await file.bytes())`) buffer content in memory at startup:
+
+- **Zero filesystem I/O** during requests - content served entirely from memory
+- **ETag support** - Automatically generates and validates ETags for caching
+- **If-None-Match** - Returns `304 Not Modified` when client ETag matches
+- **No 404 handling** - Missing files cause startup errors, not runtime 404s
+- **Memory usage** - Full file content stored in RAM
+- **Best for**: Small static assets, API responses, frequently accessed files
+
+**File routes** (`new Response(Bun.file(path))`) read from filesystem per request:
+
+- **Filesystem reads** on each request - checks file existence and reads content
+- **Built-in 404 handling** - Returns `404 Not Found` if file doesn't exist or becomes inaccessible
+- **Last-Modified support** - Uses file modification time for `If-Modified-Since` headers
+- **If-Modified-Since** - Returns `304 Not Modified` when file hasn't changed since client's cached version
+- **Range request support** - Automatically handles partial content requests with `Content-Range` headers
+- **Streaming transfers** - Uses buffered reader with backpressure handling for efficient memory usage
+- **Memory efficient** - Only buffers small chunks during transfer, not entire file
+- **Best for**: Large files, dynamic content, user uploads, files that change frequently
+
+### HTTP Caching Behavior
+
+Both route types implement HTTP caching standards but with different strategies:
+
+#### Static Routes Caching
+
+- **ETag generation**: Automatically computes ETag hash from content at startup
+- **If-None-Match**: Validates client ETag against server ETag
+- **304 responses**: Returns `304 Not Modified` with empty body when ETags match
+- **Cache headers**: Inherits any `Cache-Control` headers you provide in the Response
+- **Consistency**: ETag remains constant until server restart or route reload
+
+#### File Routes Caching
+
+- **Last-Modified**: Uses file's `mtime` for `Last-Modified` header
+- **If-Modified-Since**: Compares client date with file modification time
+- **304 responses**: Returns `304 Not Modified` when file unchanged since client's cached version
+- **Content-Length**: Automatically set based on current file size
+- **Dynamic validation**: Checks file modification time on each request
+
+#### Status Code Handling
+
+Both route types automatically adjust status codes:
+
+- **200 → 204**: Empty files (0 bytes) return `204 No Content` instead of `200 OK`
+- **200 → 304**: Successful cache validation returns `304 Not Modified`
+- **File routes only**: Missing or inaccessible files return `404 Not Found`
+
 ```ts
 const server = Bun.serve({
   static: {