feat: add IRC wire protocol listener with shared service layer

Add a traditional IRC wire protocol listener (RFC 1459/2812) on
configurable port (default :6667), sharing business logic with
the HTTP API via a new service layer.

- IRC listener: NICK, USER, PASS, JOIN, PART, PRIVMSG, NOTICE,
  TOPIC, MODE, KICK, QUIT, NAMES, LIST, WHOIS, WHO, AWAY, OPER,
  INVITE, LUSERS, MOTD, PING/PONG, CAP
- Service layer: shared logic for both transports including
  channel join (with Tier 2 checks: ban/invite/key/limit),
  message send (with ban + moderation checks), nick change,
  topic, kick, mode, quit broadcast, away, oper, invite
- BroadcastQuit uses FanOut pattern (one insert, N enqueues)
- HTTP handlers delegate to service for all command logic
- Tier 2 mode operations (+b/+i/+s/+k/+l) use service methods

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/ircserver/parser.go

@@ -0,0 +1,123 @@
+// Package ircserver implements a traditional IRC wire protocol
+// listener (RFC 1459/2812) that bridges to the neoirc HTTP/JSON
+// server internals.
+package ircserver
+
+import "strings"
+
+// Message represents a parsed IRC wire protocol message.
+type Message struct {
+	// Prefix is the optional :prefix at the start (may be
+	// empty for client-to-server messages).
+	Prefix string
+	// Command is the IRC command (e.g., "PRIVMSG", "NICK").
+	Command string
+	// Params holds the positional parameters, including the
+	// trailing parameter (which was preceded by ':' on the
+	// wire).
+	Params []string
+}
+
+// ParseMessage parses a single IRC wire protocol line
+// (without the trailing CR-LF) into a Message.
+// Returns nil if the line is empty.
+//
+// IRC message format (RFC 1459 §2.3.1):
+//
+//	[":" prefix SPACE] command { SPACE param } [SPACE ":" trailing]
+func ParseMessage(line string) *Message {
+	if line == "" {
+		return nil
+	}
+
+	msg := &Message{} //nolint:exhaustruct // fields set below
+
+	// Extract prefix if present.
+	if line[0] == ':' {
+		idx := strings.IndexByte(line, ' ')
+		if idx < 0 {
+			// Only a prefix, no command — invalid.
+			return nil
+		}
+
+		msg.Prefix = line[1:idx]
+		line = line[idx+1:]
+	}
+
+	// Skip leading spaces.
+	line = strings.TrimLeft(line, " ")
+	if line == "" {
+		return nil
+	}
+
+	// Extract command.
+	idx := strings.IndexByte(line, ' ')
+	if idx < 0 {
+		msg.Command = strings.ToUpper(line)
+
+		return msg
+	}
+
+	msg.Command = strings.ToUpper(line[:idx])
+	line = line[idx+1:]
+
+	// Extract parameters.
+	for line != "" {
+		line = strings.TrimLeft(line, " ")
+		if line == "" {
+			break
+		}
+
+		// Trailing parameter (everything after ':').
+		if line[0] == ':' {
+			msg.Params = append(msg.Params, line[1:])
+
+			break
+		}
+
+		idx = strings.IndexByte(line, ' ')
+		if idx < 0 {
+			msg.Params = append(msg.Params, line)
+
+			break
+		}
+
+		msg.Params = append(msg.Params, line[:idx])
+		line = line[idx+1:]
+	}
+
+	return msg
+}
+
+// FormatMessage formats an IRC message into wire protocol
+// format (without the trailing CR-LF).
+func FormatMessage(
+	prefix, command string,
+	params ...string,
+) string {
+	var buf strings.Builder
+
+	if prefix != "" {
+		buf.WriteByte(':')
+		buf.WriteString(prefix)
+		buf.WriteByte(' ')
+	}
+
+	buf.WriteString(command)
+
+	for i, param := range params {
+		buf.WriteByte(' ')
+
+		isLast := i == len(params)-1
+		needsColon := strings.Contains(param, " ") ||
+			param == "" || param[0] == ':'
+
+		if isLast && needsColon {
+			buf.WriteByte(':')
+		}
+
+		buf.WriteString(param)
+	}
+
+	return buf.String()
+}