§B: fix Closing-state teardown data loss (from 47d44fa)

In the Closing state the old code force-transitioned to Closed after a
SINGLE inbound packet, so any data/ACKs the peer still had in flight when we
initiated Close were dropped (Closed returns Noop for everything). Stay in
Closing and process inbound normally; handle_inbound already owns the
Close->Closed transition when it actually receives the peer's Close frame.
Also handle Tick in Closing so our own Close frame is retransmitted until
acknowledged.

Ported surgically from 47d44fa — only the Closing-state correctness fix, NOT
that commit's bundled RFC-6298 RTO / congestion rewrite (a behavioural change
to the working base) or the sent_history BTreeMap perf swap (broad hot-path
change for a perf-only gain). cargo test -p ostp-core: 36/36 incl.
test_close_sequence.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ostp-core/src/protocol.rs

@@ -207,13 +207,16 @@ impl ProtocolMachine {
                     .map(ProtocolAction::SendDatagram)
             }
             (OstpState::Closing, OstpEvent::Inbound(raw)) => {
-                // Process final in-flight packets to prevent data loss during teardown.
-                // The remote may still have data or ACKs in transit when we initiated Close.
-                let result = self.handle_inbound(raw);
-                self.state = OstpState::Closed;
-                result
+                // The remote may still have data or ACKs in transit when we initiated
+                // Close. Stay in Closing and process them; handle_inbound transitions to
+                // Closed only when it actually receives the peer's Close frame — the old
+                // code force-closed after a single inbound packet, losing in-flight data.
+                // (Ported from 0.3.x 47d44fa.)
+                self.handle_inbound(raw)
             }
             (OstpState::Established, OstpEvent::Tick) => self.handle_tick(),
+            // Retransmit our Close frame (and drain pending) while waiting for teardown.
+            (OstpState::Closing, OstpEvent::Tick) => self.handle_tick(),
             (OstpState::Closed, _) => Ok(ProtocolAction::Noop),
             (_, OstpEvent::Close) => {
                 self.state = OstpState::Closed;