add configurable keep_alive on HttpTransport and DidResolver

Co-Authored-By: Claude Opus 4.6 <[e-mail]>

src/internal/identity/did_resolver.zig

···
     plc_url: []const u8 = "https://plc.directory",
 
     pub fn init(allocator: std.mem.Allocator) DidResolver {
+        return initWithOptions(allocator, .{});
+    }
+
+    pub const Options = struct {
+        keep_alive: bool = true,
+    };
+
+    pub fn initWithOptions(allocator: std.mem.Allocator, options: Options) DidResolver {
+        var transport = HttpTransport.init(allocator);
+        transport.keep_alive = options.keep_alive;
         return .{
             .allocator = allocator,
-            .transport = HttpTransport.init(allocator),
+            .transport = transport,
         };
     }
 

src/internal/xrpc/transport.zig

···
 pub const HttpTransport = struct {
     allocator: std.mem.Allocator,
     http_client: std.http.Client,
+    keep_alive: bool = true,
 
     // 0.16: will add
     // io: ?std.Io = null,
···
             .payload = options.payload,
             .headers = headers,
             .extra_headers = extra_buf[0..extra_count],
+            .keep_alive = self.keep_alive,
         }) catch return error.RequestFailed;
 
         const body = aw.toArrayList().items;