Merge #10: refactor!: Unify PendingRequest and clean up API

d9521fe refactor!: Move client type aliases into client module (志宇)
0f4c565 refactor!: Clean up API naming, remove dead code, and restructure modules (志宇)
4509768 feat!: Unify PendingRequest via closure-based type erasure (志宇)

Pull request description:

  ## Summary

  Eliminate duplicated `AsyncPendingRequest`/`BlockingPendingRequest` 21-variant enums and clean up the public API.

  ### Type erasure (`4509768`)

  Replace the two parallel pending request enums with a single `PendingRequest` struct that uses a `Box<dyn FnOnce + Send + Sync>` handler for closure-based type erasure. This:

  - Makes `RequestTracker` (formerly `State`) non-generic
  - Collapses `AsyncBatchRequest`/`BlockingBatchRequest` into a single `BatchRequest`
  - Removes all async/blocking-specific type aliases for response channels
  - Adds `BatchRequest::request_async()` and `request_blocking()` convenience methods that create channels internally and return receivers

  ### Naming and module cleanup (`0f4c565`)

  - **Type renames**: `State` → `RequestTracker`, `SatisfiedRequest` → `CompletedRequest`, `ErroredRequest` → `FailedRequest`, `RawNotificationOrResponse` → `RawIncoming`, `MaybeBatch` → `RawOneOrMany`
  - **Method renames**: `process_incoming` → `handle_incoming`, `into_satisfied` → `into_completed`, `into_errored` → `into_failed`
  - **Dead code removed**: `BatchRequestError` (no longer referenced after the type erasure refactor)
  - **Module restructure**: merged `batch_request.rs` into `pending_request.rs`, extracted wire types from `lib.rs` into `protocol.rs`, renamed `state.rs` → `request_tracker.rs`

  ### Client module separation (`d9521fe`)

  - Moved async/blocking type aliases from `lib.rs` into `client` module
  - Deleted 4 unused aliases: `AsyncEventSender`, `AsyncRequestReceiver`, `BlockingRequestReceiver`, `BlockingEventSender`
  - `client` is now `pub mod` — non-re-exported types available via `electrum_streaming_client::client::*`
  - Only `AsyncClient`, `BlockingClient`, and their error types are re-exported at the crate root

  ## Breaking changes

  - All renamed types and methods listed above
  - `AsyncBatchRequest` / `BlockingBatchRequest` replaced by unified `BatchRequest`
  - `AsyncPendingRequest` / `BlockingPendingRequest` replaced by unified `PendingRequest`
  - `State` is now `RequestTracker` and is no longer generic
  - `AsyncRequestSender`, `AsyncEventReceiver`, `BlockingRequestSender`, `BlockingEventReceiver` moved from crate root to `client` module

ACKs for top commit:
  evanlinjin:
    self-ACK d9521fe

Tree-SHA512: f99268135aab474befe2a866a5fd9ef067c2bb3d8aef6f14ccc49abbfb79aadd1669cb340563abe159293c7d0976138a76ed962d28af3ff09b546b8f94f721a3

Author: evanlinjin

README.md

@@ -10,13 +10,13 @@ models.
 
 - **Streaming protocol support**: Handles both server-initiated notifications and responses.
 - **Transport agnostic**: Works with any I/O type implementing the appropriate `Read`/`Write` traits.
-- **Sans-IO core**: The [`State`] struct tracks pending requests and processes server messages.
+- **Sans-IO core**: The [`RequestTracker`] struct tracks pending requests and handles incoming server messages.
 - **Typed request/response system**: Strongly typed Electrum method wrappers with minimal overhead.
 
 ## Example (async with Tokio)
 
 ```rust,no_run
-use electrum_streaming_client::{AsyncClient, AsyncBatchRequest, Event};
+use electrum_streaming_client::{AsyncClient, Event};
 use tokio::net::TcpStream;
 use futures::StreamExt;
 
@@ -28,11 +28,7 @@ async fn main() -> anyhow::Result<()> {
 
     tokio::spawn(worker); // spawn the client worker task
 
-    let mut batch = AsyncBatchRequest::new();
-    let fut = batch.request(electrum_streaming_client::request::RelayFee);
-    client.send_batch(batch)?;
-    let relay_fee = fut.await?;
-
+    let relay_fee = client.send_request(electrum_streaming_client::request::RelayFee).await?;
     println!("Relay fee: {relay_fee:?}");
 
     while let Some(event) = events.next().await {