feat(story-2.1): Middleware protocol, Next type, and chain composition

CHANGELOG.md

@@ -19,5 +19,6 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 - Status-keyed exception hierarchy with plain typed fields: `ClientError`, `TransportError`, `TimeoutError`, `StatusError`, `ClientStatusError`/`ServerStatusError` bases, 9 leaf classes (`BadRequestError` … `ServiceUnavailableError`), `STATUS_TO_EXCEPTION` lookup dict (Story 1.3). `StatusError` is picklable and deep-copyable via custom `__reduce__`; `__repr__` and the summary message strip `user:pass@` userinfo from the request URL; `headers` is stored as a read-only `MappingProxyType` so caller mutations after `raise` do not bleed into the exception. `TimeoutError` multi-inherits from `builtins.TimeoutError` (revisits architecture Decision 3) so `except builtins.TimeoutError` (the form `asyncio.wait_for` raises) also catches httpware-raised timeouts.
 - `Transport` protocol (`@runtime_checkable`) and default `Httpx2Transport` adapter; `StreamResponse` placeholder for Story 4.1 protocol typing; the wire `method` is uppercased at the seam and `httpx2` exceptions (`TimeoutException`, `HTTPError`, `InvalidURL`, `CookieConflict`, and the closed-client `RuntimeError`) are mapped to `httpware.TimeoutError` / `httpware.TransportError` (with the original exception's message preserved on the mapped instance) so no `httpx2` exception escapes the library; lazy `httpx2.AsyncClient` construction is guarded by an `asyncio.Lock` so concurrent first-calls share one client; `httpx2` is confined to `src/httpware/transports/httpx2.py` (Story 1.4).
 - `ResponseDecoder` protocol (`@runtime_checkable`) and default `PydanticDecoder` adapter — single-parse-pass JSON decoding via `pydantic.TypeAdapter.validate_json(bytes)`; a module-level `@functools.lru_cache(maxsize=None)` factory (`_get_adapter`) memoizes one `TypeAdapter` per `response_model` across the process so warm-path requests pay zero adapter-construction cost; `pydantic.ValidationError` surfaces unchanged to the caller (Story 1.5).
+- `Middleware` protocol (`@runtime_checkable`) and `Next` callable type alias (`Callable[[Request], Awaitable[Response]]`); private `compose(middlewares, transport)` chain composer at `httpware._internal.chain` using a recursive closure fold with `transport.__call__` as the bottom of the chain. No exception handling inside `compose`, so `asyncio.CancelledError` and user-raised exceptions propagate untouched (Story 2.1).
 
 [Unreleased]: https://github.com/modern-python/httpware/commits/main