Move cert noise calibration into doppelganger scout

Instead of a separate cert_probe.go that duplicates the scout's TLS
connection logic, measure the cert chain size directly from the same
HTTPS connections the scout already makes.

Changes:
- Extend ScoutConnResult with payloadLen field
- Add Write interception to ScoutConn for handshake boundary detection
- Scout.learn() now computes cert size (sum of ApplicationData between
  CCS and first client Write) alongside inter-record durations
- Ganger aggregates cert sizes across raids and exposes NoiseParams()
  via atomic pointer for lock-free reads from proxy goroutines
- Proxy reads NoiseParams from Ganger on each handshake instead of
  probing at startup
- Remove cert_probe.go, disk cache, and related config options
  (noise-cache-path, noise-cache-ttl, noise-probe-count)

Falls back to legacy 2500-4700 range until the first scout raid
completes (typically within 1-2 seconds of startup).

mtglib/internal/doppel/scout_conn_collected.go

@@ -9,21 +9,32 @@ const (
 type ScoutConnResult struct {
 	timestamp  time.Time
 	recordType byte
+	payloadLen int
 }
 
 type ScoutConnCollected struct {
-	data []ScoutConnResult
+	data       []ScoutConnResult
+	writeIndex int // index at which client first wrote post-handshake data; -1 if not set
 }
 
-func (s *ScoutConnCollected) Add(record byte) {
+func (s *ScoutConnCollected) Add(record byte, payloadLen int) {
 	s.data = append(s.data, ScoutConnResult{
 		timestamp:  time.Now(),
 		recordType: record,
+		payloadLen: payloadLen,
 	})
 }
 
+// MarkWrite records the current data length as the handshake boundary.
+func (s *ScoutConnCollected) MarkWrite() {
+	if s.writeIndex < 0 {
+		s.writeIndex = len(s.data)
+	}
+}
+
 func NewScoutConnCollected() *ScoutConnCollected {
 	return &ScoutConnCollected{
-		data: make([]ScoutConnResult, 0, ScoutConnCollectedPreallocSize),
+		data:       make([]ScoutConnResult, 0, ScoutConnCollectedPreallocSize),
+		writeIndex: -1,
 	}
 }