feat(infra): tipos ConfigError y ConfigValidation + funciones puras Go (validate, merge, dump)

- ConfigError y ConfigValidation como tipos producto con sus .md en types/infra/
- config_validate: validacion con tags required/format/min/max/oneof via reflection
- config_merge: merge no-mutante de map[string]string con precedencia de override
- config_dump: serializacion de structs a map con mascara *** para campos secret
- 17 tests unitarios, todos PASS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/config_merge.go

@@ -0,0 +1,16 @@
+package infra
+
+// ConfigMerge merges two string maps, with override taking precedence over base.
+// Keys present in both maps take the value from override.
+// Keys present only in base are kept. Keys present only in override are added.
+// Neither input map is mutated — a new map is returned.
+func ConfigMerge(base, override map[string]string) map[string]string {
+	result := make(map[string]string, len(base)+len(override))
+	for k, v := range base {
+		result[k] = v
+	}
+	for k, v := range override {
+		result[k] = v
+	}
+	return result
+}