[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/peer/handshaker.go

@@ -171,10 +171,11 @@ func (h *Handshaker) handleRemoteOffer(remoteOfferAnswer OfferAnswer) {
 	// Derive+store the KEM PSK (inside sendAnswer's AnswerPayload) BEFORE bringing up the
 	// connection: the relay/ICE workers configure the WG endpoint, which pulls the PSK
 	// for the first handshake. Notifying them first would race the KEM exchange and hand
-	// the first handshake a not-yet-derived key.
+	// the first handshake a not-yet-derived key. A failure to signal the answer is
+	// transient (the peer retries), so still bring the local transport up over relay/ICE
+	// instead of aborting the whole setup.
 	if err := h.sendAnswer(&remoteOfferAnswer); err != nil {
 		h.log.Errorf("failed to send remote offer confirmation: %s", err)
-		return
 	}
 	h.notifyListeners(&remoteOfferAnswer)
 }