docs(http): clarify serializeStream return type and granularity

The Handler Interface table typed `serializeStream` as returning a WHATWG
`ReadableStream`. Verified against harper origin/main: every built-in handler
returns a Node.js `Readable` (`streamAsJSON` -> `JSONStream extends Readable`,
`Readable.from(...)`, `EncoderStream`, `toCsvStream`), and the call site in
`server/serverHelpers/contentTypes.ts` pipes the return value through
`createBrotliCompress()` and hands it to the HTTP layer as the response body.

Also record the granularity: `serializeStream` is invoked once per response
with the entire iterable, and only when the response body is an iterable or
async iterable. Per-chunk serialization is `serialize`, which the streaming
handlers call for each message.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Ethan-Arrowood

reference/http/api.md

@@ -434,12 +434,12 @@ contentTypes.set('text/xml', {
 
 ### Handler Interface
 
-| Property                | Type                                      | Description                                                                               |
-| ----------------------- | ----------------------------------------- | ----------------------------------------------------------------------------------------- |
-| `serialize(data)`       | `(any) => Buffer \| Uint8Array \| string` | Serialize data for a response                                                             |
-| `serializeStream(data)` | `(any) => ReadableStream`                 | Serialize as a stream (for async iterables or large data)                                 |
-| `deserialize(buffer)`   | `(Buffer \| string) => any`               | Deserialize an incoming request body. String for `text/*` types, Buffer for binary types. |
-| `q`                     | number (0–1)                              | Quality indicator for content negotiation. Defaults to `1`.                               |
+| Property                | Type                                      | Description                                                                                                                                                                                                                                                            |
+| ----------------------- | ----------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `serialize(data)`       | `(any) => Buffer \| Uint8Array \| string` | Serialize data for a response                                                                                                                                                                                                                                          |
+| `serializeStream(data)` | `(any) => Readable`                       | Serialize a streaming response body. Called once per response with the whole iterable (not once per chunk), and only when the response body is an iterable or async iterable. Returns a Node.js [`Readable`](https://nodejs.org/api/stream.html#class-streamreadable). |
+| `deserialize(buffer)`   | `(Buffer \| string) => any`               | Deserialize an incoming request body. String for `text/*` types, Buffer for binary types.                                                                                                                                                                              |
+| `q`                     | number (0–1)                              | Quality indicator for content negotiation. Defaults to `1`.                                                                                                                                                                                                            |
 
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