[GoDeskio]

Backlog #13 (MSP model).

• src/tenancy.rs: hierarchical namespaces. A child's limits can't exceed its parent's.
• Key quotas count the whole subtree.
• A per-namespace token bucket is charged atomically up the ancestor chain.
• The clock is injected, so tests are deterministic.
• Tests: hierarchy validation, quota denials at child and parent level, rate limit and refill.