refactor(client): remove redundant timestamp parameter from poll_timeouts

Use internal monotonic time by default and introduce poll_timeouts_at for callers requiring a specific sampled Instant. This simplifies the API for standard usage while maintaining flexibility for external event loops.

crates/irtt-client/tests/server_fill.rs

@@ -1,7 +1,7 @@
 mod support;
 
 use irtt_client::{
-    Client, ClientConfig, ClientError, ClientEvent, ClientTimestamp, NegotiationPolicy,
+    Client, ClientConfig, ClientError, ClientEvent, NegotiationPolicy,
 };
 use irtt_proto::{echo_packet_len, Params, TimestampFields};