docs(handler/serial): add comprehensive English comments to all files

Add package-level architecture documentation describing two modes
(hop-based forwarding and direct proxy), two-tier dialing strategy
in forwardSerial, recorderConn per-packet logging with direction
convention, and metadata parsing details. No code changes.

handler/serial/metadata.go

@@ -7,10 +7,19 @@ import (
 	mdutil "github.com/go-gost/x/metadata/util"
 )
 
+// metadata holds the parsed configuration for the serial handler.
+// It is populated from the handler's metadata map during Init().
 type metadata struct {
+	// timeout is the per-read timeout applied when opening a serial port
+	// directly (not through the router chain). It controls how long the
+	// serial port blocks waiting for data before returning to the caller.
+	// A value of 0 means blocking indefinitely (no timeout).
 	timeout time.Duration
 }
 
+// parseMetadata extracts typed configuration from the generic metadata map.
+// Supported keys: "timeout", "serial.timeout" (both accept duration strings
+// like "5s" or integer seconds).
 func (h *serialHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.timeout = mdutil.GetDuration(md, "timeout", "serial.timeout")
 	return