[client] pqkem: address review findings on convergence, strict parsing and status

Bot review (CodeRabbit, cubic) on PR #7098 surfaced several real defects:

- Strict() parsed the raw env value instead of the normalized one, so a
  mixed-case NB_PQ_MLKEM_STRICT such as "tRuE" silently disabled fail-closed
  mode. Parse the lower-cased value, matching Enabled().
- OnDataPathMessage labeled a data-path answer as "signal" in logs, mislabeling
  every rotation answer. Use the data-path label.
- A failed sendAnswer aborted the whole connection setup, skipping the relay/ICE
  listeners; a transient signalling failure now still brings the local transport
  up (the peer retries the answer).
- processOffer left the reserved exchange slot in place when Respond failed, so
  every retransmission of that offer was dropped forever. Clear the reservation
  on a pre-commit error so a retry can derive again.
- Dropped a no-op time.Since that implied a convergence-latency metric that was
  never recorded, and the now-unused startedAt field.
- The strict-mode status line asserted active blocking even for a peer that is
  simply offline; reword it to state that no PSK is established yet.
- conn_test used t.Fatalf for conditions under test; use assert.

client/internal/pqkem/convergence.go

@@ -67,7 +67,6 @@ func (m *Manager) startExchangeLocked(remoteID RemoteID, viaSignal bool, ackID E
 	m.exchanges[remoteID] = &exchangeCtl{
 		id:        id,
 		state:     stateAwaitingAnswer,
-		startedAt: time.Now(),
 		cancel:    cancel,
 		lastSent:  raw,
 		initiator: init,
@@ -118,11 +117,19 @@ func (m *Manager) processOffer(remoteID RemoteID, o *OfferMsg, via string) ([]by
 		return last, nil
 	}
 	// Reserve the slot so a concurrent duplicate offer bails.
-	m.exchanges[remoteID] = &exchangeCtl{id: o.ExchangeID, state: stateReserved, startedAt: time.Now()}
+	m.exchanges[remoteID] = &exchangeCtl{id: o.ExchangeID, state: stateReserved}
 	m.mu.Unlock()
 
 	answerBytes, psk, err := Respond(o.KEMOffer, m.binding(remoteID))
 	if err != nil {
+		// Respond failed before the PSK was committed: clear the reservation so a
+		// retransmission of this offer (a transient or malformed first packet) can retry
+		// instead of hitting the stuck reserved slot forever.
+		m.mu.Lock()
+		if cur := m.exchanges[remoteID]; cur != nil && cur.id == o.ExchangeID && cur.state == stateReserved {
+			delete(m.exchanges, remoteID)
+		}
+		m.mu.Unlock()
 		return nil, err
 	}
 	raw, err := (&AnswerMsg{ExchangeID: o.ExchangeID, KEMAnswer: answerBytes}).Encode()
@@ -235,7 +242,6 @@ func (m *Manager) ackConverged(remoteID RemoteID, ackID ExchangeID) {
 	delete(m.exchanges, remoteID)
 	m.established[remoteID] = true
 	m.failures[remoteID] = 0
-	_ = time.Since(ex.startedAt) // convergence latency (metrics hook, later step)
 	m.mu.Unlock()
 
 	m.trace("pqkem: previous exchange confirmed by ack", "peer", remoteID, "exchange", idHex(ackID))

client/internal/pqkem/env.go

@@ -43,15 +43,16 @@ const EnvStrict = "NB_PQ_MLKEM_STRICT"
 // Strict reports whether strict (fail-closed) mode is enabled via the environment.
 // An empty or unrecognized value is treated as disabled (opportunistic).
 func Strict() bool {
-	switch strings.ToLower(strings.TrimSpace(os.Getenv(EnvStrict))) {
+	raw := strings.ToLower(strings.TrimSpace(os.Getenv(EnvStrict)))
+	switch raw {
 	case "on":
 		return true
 	case "", "off":
 		return false
 	}
-	enabled, err := strconv.ParseBool(strings.TrimSpace(os.Getenv(EnvStrict)))
+	enabled, err := strconv.ParseBool(raw)
 	if err != nil {
-		log.Warnf("failed to parse %s value %q: %v", EnvStrict, os.Getenv(EnvStrict), err)
+		log.Warnf("failed to parse %s value %q: %v", EnvStrict, raw, err)
 		return false
 	}
 	return enabled

client/internal/pqkem/manager.go

@@ -80,7 +80,6 @@ const (
 type exchangeCtl struct {
 	id         ExchangeID
 	state      exchangeState
-	startedAt  time.Time
 	cancel     context.CancelFunc
 	lastSent   []byte
 	initiator  *Initiator
@@ -402,7 +401,7 @@ func (m *Manager) OnDataPathMessage(remoteID RemoteID, raw []byte) error {
 		}
 		return m.pushDataPath(remoteID, answer)
 	case MsgAnswer:
-		return m.processAnswer(remoteID, msg.(*AnswerMsg), viaSignalLabel)
+		return m.processAnswer(remoteID, msg.(*AnswerMsg), viaDataPathLabel)
 	default:
 		return fmt.Errorf("unhandled data-path message type %d from %s", typ, remoteID)
 	}