The SDK had two unrelated XRPC exception types (the client's AtProtoHttpException,
an HttpRequestException, and the server's XrpcException), nine exceptions deriving
straight from Exception, five copies of the error-body parser, and callers matching
error names as string literals. There is now one hierarchy under AtProtoException,
one XrpcException for both client and server, XrpcErrors constants, and one reader.

The transport was unsafe to share: it wrote HttpClient.BaseAddress (which throws
after the first request, breaking SetPdsUrl and ApplyOAuthSessionAsync on reused
clients), appended a User-Agent token to the shared default headers per client,
kept proxy/labeler headers in global setters, and dropped them without a session.
It now holds its service URL, sets every header per request, and takes per-call
XrpcCallOptions. It also streams JSON instead of buffering it, sends through a
shared decompressing handler with a bounded connection lifetime, caps rate-limit
waits, formats query values culture-invariantly, rewinds uploads to the caller's
position, and returns downloads as a disposable XrpcStreamResponse.

AtProtoJsonDefaults.FormatTimestamp gets the same invariant-culture fix: under a
Thai locale every createdAt the SDK stamped carried a Buddhist-calendar year.

XrpcClient becomes internal: nothing public exposed it, and #122 adds an explicit
transport seam for third-party sub-clients.

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

@@ -0,0 +68,4 @@
+
+    /// <inheritdoc cref="NormalizeBaseUrl(string)"/>
+    internal static Uri NormalizeBaseUrl(Uri url) =>
+        url.AbsolutePath.EndsWith('/') ? url : new Uri(url.AbsoluteUri.TrimEnd('/') + "/", UriKind.Absolute);

@@ -725,0 +618,4 @@
+    }
+
+    // ──────────────────────────────────────────────────────────
+    //  Types