feat: add --type-map to override SQL-to-Go types in bun and gorm writers

Adds WriterOptions.TypeMappings and a repeatable --type-map sqltype=gotype
flag. Defaults are unchanged when no mapping is given. Closes #36 (bun/gorm).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

pkg/writers/typemap.go

@@ -0,0 +1,46 @@
+package writers
+
+import (
+	"fmt"
+	"strings"
+
+	"git.warky.dev/wdevs/relspecgo/pkg/pgsql"
+)
+
+// ParseTypeMappings parses "sqltype=gotype" entries (as given to --type-map)
+// into a map keyed by the canonical lower-case SQL base type, so that
+// "VARCHAR", "character varying" and "varchar(50)" all address one entry.
+// It returns nil for empty input.
+func ParseTypeMappings(entries []string) (map[string]string, error) {
+	if len(entries) == 0 {
+		return nil, nil
+	}
+	out := make(map[string]string, len(entries))
+	for _, entry := range entries {
+		sqlType, goType, ok := strings.Cut(entry, "=")
+		sqlType, goType = strings.TrimSpace(sqlType), strings.TrimSpace(goType)
+		if !ok || sqlType == "" || goType == "" {
+			return nil, fmt.Errorf("invalid type mapping %q: expected sqltype=gotype", entry)
+		}
+		out[pgsql.CanonicalizeBaseType(pgsql.ExtractBaseTypeLower(sqlType))] = goType
+	}
+	return out, nil
+}
+
+// LookupTypeMapping returns the user-configured override for baseType, which
+// the caller must already have canonicalized.
+func LookupTypeMapping(mappings map[string]string, baseType string) (string, bool) {
+	goType, ok := mappings[baseType]
+	return goType, ok
+}
+
+// ApplyTypeMapping wraps an overridden Go type for nullability: NOT NULL uses
+// the type verbatim; nullable columns get a pointer prefix unless the type is
+// already a pointer, slice, map or interface.
+func ApplyTypeMapping(goType string, notNull bool) string {
+	if notNull || strings.HasPrefix(goType, "*") || strings.HasPrefix(goType, "[]") ||
+		strings.HasPrefix(goType, "map[") || goType == "any" || goType == "interface{}" {
+		return goType
+	}
+	return "*" + goType
+}