Fix short flags, AllowTCP wiring, invalid root server error, add config tests

- Register -v/-q short flag aliases sharing the same bool variable as
  --verbose/--quiet so both forms work identically
- Register -d and -dd flags; use ParseDebugLevel() helper so -d sets
  Debug=1 and -dd sets Debug=2; remove the broken strings.Count approach
- Add AllowTCP field to dns.QueryConfig; guard TCP truncation fallback in
  QueryWithExchange and IterativeQueryWithExchange behind cfg.AllowTCP;
  wire cfg.AllowTCP from CLI config into QueryConfig in main.go
- ParseRootServer() now returns ErrInvalidRootServer instead of (nil,nil)
  when the string is non-empty but net.ParseIP fails
- Add internal/config/config_test.go covering ParseQueryType, ParseUDPSize,
  Validate, GetDomain, ParseRootServer, ParseDebugLevel, and the
  --always-tcp/--allow-tcp cross-check
- Add TestQueryNoTCPFallbackWhenDisabled to dns/query_test.go
- Update existing truncation tests to set AllowTCP:true
- Add TODO comments on display flags not yet wired to output layer

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/config/config.go

@@ -11,12 +11,13 @@ import (
 )
 
 var (
-	ErrInvalidQueryType    = errors.New("invalid query type")
-	ErrInvalidUDPSize      = errors.New("UDP size must be between 512 and 4096")
-	ErrInvalidMaxDepth     = errors.New("max depth must be between 1 and 100")
-	ErrInvalidRetries      = errors.New("retries must be between 0 and 10")
-	ErrMissingDomain       = errors.New("domain is required")
+	ErrInvalidQueryType     = errors.New("invalid query type")
+	ErrInvalidUDPSize       = errors.New("UDP size must be between 512 and 4096")
+	ErrInvalidMaxDepth      = errors.New("max depth must be between 1 and 100")
+	ErrInvalidRetries       = errors.New("retries must be between 0 and 10")
+	ErrMissingDomain        = errors.New("domain is required")
 	ErrAlwaysTCPRequiresTCP = errors.New("--always-tcp requires --allow-tcp")
+	ErrInvalidRootServer    = errors.New("invalid root server IP address")
 )
 
 type Config struct {
@@ -138,7 +139,23 @@ func (c *Config) ParseRootServer() (net.IP, error) {
 	if c.RootServer == "" {
 		return nil, nil
 	}
-	return net.ParseIP(c.RootServer), nil
+	ip := net.ParseIP(c.RootServer)
+	if ip == nil {
+		return nil, fmt.Errorf("%w: %s", ErrInvalidRootServer, c.RootServer)
+	}
+	return ip, nil
+}
+
+// ParseDebugLevel returns the debug verbosity level from the -d and -dd flag values.
+// dd=true → 2 (library debug), d=true → 1 (application debug), neither → 0.
+func ParseDebugLevel(d, dd bool) int {
+	if dd {
+		return 2
+	}
+	if d {
+		return 1
+	}
+	return 0
 }
 
 func PrintUsage() {