General Upgrade, NEW: WebServers, Better Docs

docs/ERRORS.md

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+# Error Reference
+
+MTP reports codec failures separately from connection and transport failures.
+
+## CodecError
+
+| Variant | Meaning |
+| --- | --- |
+| `UnknownVersion` | A codec was asked to use an unsupported protocol version. |
+| `UnknownCommunicationType` | A communication type has no mapping in the selected type map. |
+| `UnknownDataType` | A data type has no mapping in the selected type map. |
+| `ReservedCommunicationType` | An application attempted to use a reserved communication type ID. |
+| `InvalidEncoding` | Bytes do not match the MTP value or frame format. |
+| `TooManyEntries` | A serialized value or frame exceeds its representable size. |
+| `CryptoFailed` | Signing, verification, encryption, or decryption failed while encoding or decoding. |
+| `MissingField` | A required typed field is absent. |
+
+An application should select a `TypeMap` for the negotiated version and treat `UnknownCommunicationType` and `UnknownDataType` as a type-map compatibility failure. Do not send the unmapped variant again on that connection.
+
+Retry guidance: retry `ConnectionLost`, `ConnectingError`, and transient stream errors after applying backoff. Correct the request before retrying `MessageTooLarge`, `InvalidEncoding`, `MissingField`, and type-map errors.
+Retry `AuthenticationFailed` only after changing credentials or host policy;
+repeating the same proof does not repair a validation failure.
+
+## CommunicationError
+
+| Variant | Typical cause |
+| --- | --- |
+| `UseAfterClosed` | A send or receive operation ran after the connection handle was closed. |
+| `ClosedLocally` | The local endpoint initiated shutdown. |
+| `ClosedByPeer` | The peer closed the connection. |
+| `ConnectionLost` | The connection ended without a normal close. |
+| `ParseCommunicationValue` or `ParseError` | An incoming frame or certificate could not be parsed. |
+| `Encode` | An outgoing value could not be serialized. |
+| `MessageTooLarge` | A frame exceeds `Policy::max_message_size` or the handshake limit. |
+| `StreamClosed` or `StreamError` | A QUIC stream ended or returned an I/O error. |
+| `ConnectingError` or `ConnectionError` | The endpoint could not establish or maintain QUIC. |
+| `AuthenticationFailed` | A login, registration, signature, nonce, or host response failed validation. |
+| `CertificateParseFailed` or `CertificateLoadFailed` | TLS certificate input is malformed or unavailable. |
+| `CryptoProviderInstallFailed` | The native TLS crypto provider could not be installed. |
+| `Other` | A component returned an error without a more specific variant. |
+
+Native builds may expose additional variants wrapping QUIC and WebTransport errors. WASM builds expose the transport-independent subset.
+
+## Authentication Rejections
+
+The host reports unsupported or missing protocol versions through `AcceptError`. Authentication failures return `AcceptError::AuthenticationFailed` after the host sends a rejected handshake response. The authentication flow and its signed fields are defined in [Security](SECURITY.md).