refactor(bigtable): 1x heartbeat + drop `active := 1` code smell

Two follow-ups after the java-bigtable heartbeat cross-check:

1) handleSessionParameters: drop 3x multiplier on nextHeartbeatDeadline
   (already done in resetHeartbeatDeadline in the prior PR-2 commit).
   Now 1 * interval, matching Java's SessionImpl.java:440 which sets
   nextHeartbeat = now + heartbeatInterval on every reset. Wake-comment
   updated to reflect the new arithmetic.

2) heartBeatLoop:
   - Delete the `active := 1 // multiplex=1; kept as %d so a future >1
     stays greppable` code smell. Inline `1` was documenting itself;
     the variable existed only for its comment. Drop it — when
     multiplex > 1 lands, the caller will read `s.activeVRPCCount()`
     (or similar), not this stale scaffolding.
   - `lastFrameAge = interval - remaining` (was `3*interval - remaining`)
     to reflect the new 1x deadline.
   - Debugf/recordEvent format strings drop the `in_flight=%d` field
     that was carrying the constant `1`.

Test adjustments:
- TestHandleSessionParameters_UpdatesIntervalAndDeadline: expect
  deadline ~= before+interval (1x) instead of before+3*interval.
- TestHeartBeatLoop_HeartbeatsKeepInflightVRPCAlive: seed deadline
  at now+30ms (1x) instead of now+90ms (was 3x).

bigtable/internal/transport/session_lifecycle.go

diff --git a/bigtable/internal/transport/session_lifecycle.go b/bigtable/internal/transport/session_lifecycle.go
index 255a794..0591774 100644
--- a/bigtable/internal/transport/session_lifecycle.go
+++ b/bigtable/internal/transport/session_lifecycle.go
@@ -335,11 +335,11 @@
 		return
 	}
 	s.heartbeatIntervalNano.Store(int64(interval))
-	s.nextHeartbeatDeadlineNano.Store(time.Now().Add(3 * interval).UnixNano())
+	s.nextHeartbeatDeadlineNano.Store(time.Now().Add(interval).UnixNano())
 	// Wake the watchdog: this is the sole path that changes the interval
 	// itself (not just the deadline), so the Timer must re-evaluate
-	// against 3×interval instead of the initialHeartbeatGrace bootstrap
-	// it was armed to at NewSession.
+	// against the new interval instead of the initialHeartbeatGrace
+	// bootstrap it was armed to at NewSession.
 	s.wakeHeartbeatLoop()
 }
 
@@ -551,20 +551,20 @@
 			timer.Reset(interval)
 			continue
 		}
-		active := 1 // multiplex=1; kept as %d so a future >1 stays greppable.
 		remaining := time.Until(time.Unix(0, s.nextHeartbeatDeadlineNano.Load()))
-		// last-frame age = (deadline - now) inverted into "how long
-		// since the last frame extended us" = 3*interval - remaining.
-		lastFrameAge := 3*interval - remaining
+		// last-frame age = interval - remaining (deadline is set to
+		// now+interval on every inbound/outbound frame via
+		// resetHeartbeatDeadline).
+		lastFrameAge := interval - remaining
 
 		if remaining > 0 {
 			// Deadline was pushed out while we were sleeping; re-arm.
 			// Only record when last_frame_age has crossed one interval —
 			// otherwise every healthy session would spam the UI ring
-			// buffer ~3x/second and drown out close/missed events.
+			// buffer and drown out close/missed events.
 			if lastFrameAge >= interval {
-				s.recordEvent("hb-alive", "in_flight=%d last_frame_age=%v remaining=%v interval=%v",
-					active, lastFrameAge, remaining, interval)
+				s.recordEvent("hb-alive", "last_frame_age=%v remaining=%v interval=%v",
+					lastFrameAge, remaining, interval)
 			}
 			timer.Reset(remaining)
 			continue
@@ -574,10 +574,10 @@
 		// ForceClose so we have a definitive marker even if downstream
 		// cancel races.
 		recordDebugTag(tagSessionHeartbeatMissed)
-		s.debugf("heartbeat MISSED — forcing close in_flight=%d last_frame_age=%v interval=%v",
-			active, lastFrameAge, interval)
-		s.recordEvent("hb-missed", "in_flight=%d last_frame_age=%v interval=%v",
-			active, lastFrameAge, interval)
+		s.debugf("heartbeat MISSED — forcing close last_frame_age=%v interval=%v",
+			lastFrameAge, interval)
+		s.recordEvent("hb-missed", "last_frame_age=%v interval=%v",
+			lastFrameAge, interval)
 		s.ForceClose(&spb.CloseSessionRequest{
 			Reason:      spb.CloseSessionRequest_CLOSE_SESSION_REASON_MISSED_HEARTBEAT,
 			Description: "client terminated session due to missed server heartbeats",

bigtable/internal/transport/session_lifecycle_test.go

diff --git a/bigtable/internal/transport/session_lifecycle_test.go b/bigtable/internal/transport/session_lifecycle_test.go
index 7d3a41c..3d1aa49 100644
--- a/bigtable/internal/transport/session_lifecycle_test.go
+++ b/bigtable/internal/transport/session_lifecycle_test.go
@@ -191,7 +191,10 @@
 	if gotInterval != 2*time.Second {
 		t.Errorf("heartbeatInterval = %v, want 2s", gotInterval)
 	}
-	wantMin := before.Add(5 * time.Second)
+	// Deadline should be about before+interval (1x, matching
+	// resetHeartbeatDeadline). Below-interval floor tolerates the
+	// scheduling gap between capturing `before` and the atomic store.
+	wantMin := before.Add(time.Second)
 	if gotDeadline.Before(wantMin) {
 		t.Errorf("nextHeartbeatDeadline = %v, want >= %v", gotDeadline, wantMin)
 	}
@@ -432,7 +435,7 @@
 func TestHeartBeatLoop_HeartbeatsKeepInflightVRPCAlive(t *testing.T) {
 	s, _ := makeActive(t, SessionHooks{})
 	s.heartbeatIntervalNano.Store(int64(30 * time.Millisecond))
-	s.nextHeartbeatDeadlineNano.Store(time.Now().Add(90 * time.Millisecond).UnixNano()) // 3 * interval
+	s.nextHeartbeatDeadlineNano.Store(time.Now().Add(30 * time.Millisecond).UnixNano()) // 1 * interval
 	s.setSlotForTest(&vrpcImpl{id: 1, resultChan: make(chan vrpcResult, 1)})
 
 	ctx, cancel := context.WithCancel(context.Background())