feat(pcap): complete replay system with filtering, metadata, and live replay

Wire ExcludeOpcodes config into RecordingConn so configured opcodes
(e.g. ping, nop, position) are filtered at record time. Add padded
metadata with in-place PatchMetadata to populate CharID/UserID after
login. Implement --mode replay using protbot's encrypted connection
with timing-aware packet sending, auto-ping response, concurrent
S→C collection, and byte-level payload diff reporting.

network/pcap/format.go

@@ -12,6 +12,11 @@ const (
 
 	// HeaderSize is the fixed size of the file header in bytes.
 	HeaderSize = 32
+
+	// MinMetadataSize is the minimum metadata block size in bytes.
+	// Metadata is padded to at least this size to allow in-place patching
+	// (e.g., adding CharID/UserID after login).
+	MinMetadataSize = 512
 )
 
 // Direction indicates whether a packet was sent or received.