feat(tcp): a failed send or receive says which errno it failed with

Send() and the receives threw a bare std::runtime_error with the
strerror text, so a caller that reports the cause the way Python does -
"the connection broke ([Errno 104] Connection reset by peer)", which a
language model reads in Crafter.Review's SQL helper output - could not
tell the code. They throw SocketErrorException now, as a failed connect
already does: kind System, the errno, and the same message, still a
std::runtime_error. A receive timeout stays SocketTimeoutException and
a closed peer SocketClosedException.

ShouldReportIoErrors: a peer that resets the connection (SO_LINGER 0)
fails the next receive with ECONNRESET and a send into it with EPIPE
or ECONNRESET.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -53,7 +53,7 @@ client.Send("Hello World", 11);
 std::vector<char> data = client.RecieveSync();
 ```
 
-The host is resolved for IPv4 and IPv6 alike and every address it resolves to is tried in turn, as Python's `socket.create_connection` does, so `localhost` reaches a server that listens on `::1` only. The optional third argument is a connect timeout, applied to each address on its own. A failed resolve or connect throws `SocketErrorException`: still a `std::runtime_error` with the same message, but carrying a `kind` (`SocketErrorKind::System` for an errno such as `ECONNREFUSED`, `SocketErrorKind::Resolver` for a getaddrinfo result such as `EAI_NONAME`) and the `code`, so a caller can report the cause without parsing the text.
+The host is resolved for IPv4 and IPv6 alike and every address it resolves to is tried in turn, as Python's `socket.create_connection` does, so `localhost` reaches a server that listens on `::1` only. The optional third argument is a connect timeout, applied to each address on its own. A failed resolve, connect, send or receive throws `SocketErrorException`: still a `std::runtime_error` with the same message, but carrying a `kind` (`SocketErrorKind::System` for an errno such as `ECONNREFUSED`, `SocketErrorKind::Resolver` for a getaddrinfo result such as `EAI_NONAME`) and the `code`, so a caller can report the cause without parsing the text.
 
 ```cpp
 try {
@@ -327,6 +327,7 @@ The library includes tests covering:
 - Mutual TLS and the raw stream (`ShouldRequireClientCertificateHTTPS1`) — a client certificate the listener's CA vouches for is served and one absent is refused; `TLSStream` is also driven directly with hand-written HTTP/1.1 to keep `:TLS` usable without `:ClientHTTP1` on top
 - Dual-stack connect (`ShouldConnectDualStack`) — `localhost` reaches a server on `::1` only and one on `127.0.0.1` only, an IPv6 literal connects over IPv6, and the last failure is what is thrown when no address answers; skips on a host without IPv6 loopback
 - Connect errors (`ShouldReportConnectErrors`) — a refused connect and an unresolvable name throw `SocketErrorException` with their errno or getaddrinfo code and the message ClientTCP always threw, and still reach a `std::runtime_error` handler
+- Send and receive errors (`ShouldReportIoErrors`) — a connection the peer resets fails the next receive, and a send into it, with `SocketErrorException` carrying ECONNRESET or EPIPE under the plain strerror text a receive and a send always threw
 - Loopback listeners (`ShouldBindLoopbackEphemeral`) — a listener on `127.0.0.1` at port 0 reports its port, takes loopback clients and refuses them through the host's other address; `::1` works too, a name is refused, and the port-only constructor still listens on every interface
 - HTTP/1.1 timeouts (`ShouldTimeoutSilentHTTP1`) — a server that accepts and never answers fails after `timeout`, and a connect nobody answers after `connectTimeout`
 - Redirects (`ShouldFollowRedirectsHTTP1`) — relative and `..` Locations, an http-to-https hop, the 4-repeat and 10-URL limits, a redirect without Location, POST after 303 and 307, percent-encoding a Location, and the URLs `FetchHTTP1` refuses