RFC 0002: Epic / Tracking issue for affinity model

[rmanibus]

Context

Track completion and rollout for RFC 0002 (Affinity Model).

RFC: rfcs/0002-affinity-model.md

Goal

Improve HAMT locality for incremental backups by using affinity-aware routing keys so updates in the same parent directory rewrite far fewer metadata nodes.

Scope

• Introduce AffinityKey-style HAMT key routing behavior.
• Integrate affinity routing through backup scan/write paths.
• Add benchmarks and benchmark automation for locality impact.
• Update docs/spec and RFC references.

Linked PRs

• feat: affinity model for locality-preserving HAMT keys (RFC 0002) #61 implementation
• docs: affinity model spec update (RFC 0002) #62 spec/docs follow-up

[rmanibus]

Implementation PR: #61
Docs PR: #62