refactor(client): capture send timestamp before socket call

Record the send timestamp immediately before the socket operation to include local enqueue overhead in RTT measurements, providing more conservative latency data for consumers.

docs/IMPLEMENTATION_PLAN.md

@@ -546,8 +546,8 @@ Steps:
 1. Assert Active phase
 2. Check if duration exceeded (if finite mode) → if so, transition to Draining
 3. Encode echo request with current sequence number, connection token
-4. Call `socket.send()`
-5. Capture send timestamp (Instant + SystemTime) immediately after send returns
+4. Capture send timestamp (Instant + SystemTime) immediately before send call
+5. Call `socket.send()`
 6. Insert PendingProbe into pending map
 7. Increment sequence counters
 8. Return any events (e.g., eviction-triggered loss events from bounded map)
@@ -649,7 +649,7 @@ No signal handling in `Client`. Caller controls lifecycle by:
 - Maintains absolute send schedule: `scheduled_time(seq) = session_start + seq * interval`
 - Prepares echo request packets
 - Sends packets at scheduled times
-- Captures send timestamps immediately after send
+- Captures send timestamps immediately before send call
 - Sends compact `SentRecord { wire_seq, logical_seq, sent_at_mono, sent_at_wall }` to coordinator
 - Must NOT perform: event fanout, stats, subscriber delivery, blocking coordination
 
@@ -1861,7 +1861,7 @@ Require `irtt` server binary in PATH. Gated behind `--features interop`.
 
 **Blocks implementation:** No. This is an implementation choice.
 
-**Recommended behavior:** Capture `Instant::now()` + `SystemTime::now()` immediately after `send()` returns. This is what the spec recommends and is the simplest correct approach.
+**Recommended behavior:** Capture `Instant::now()` + `SystemTime::now()` immediately before `send()`. This ensures the measured RTT includes local send/enqueue overhead, which is more conservative for latency-control consumers like sqm-autorate-rust. The receive timestamp remains captured immediately after `recv()` returns.
 
 ### Q3: RTT When Server Processing > Raw RTT (Section 19.12)