refactor(remote): make parse-mutation support arbitrary nesting depth

[chickendreanso]

• Replace hardcoded two-case cond in parse-mutation with recursive walk
• Single-key keyword maps are traversed until reaching the mutation leaf {nil-or-map value}
• Supports patterns like {:a {:role/member {:posts {nil data}}}} (any depth)
• All existing behavior preserved — 5 new RCT assertions added for deep nesting