Auto-load tiered checkpoints for MatFormer tiers (remote + local)

[plugyawn]

Problem

--matformer-load-strategy auto only checks a local -tier{N} path. If the checkpoint is remote (HF), it downloads the universal repo and does not attempt a tiered repo. Small-tier nodes can OOM unless operators manually provide tiered checkpoints.

Refs:

• shared/client/src/state/init.rs (local-only tier path detection)

Expected

Auto strategy should prefer tiered checkpoints when available (local or remote), or offer an explicit override to point to tiered repos.

Possible Approach

• Add --matformer-tier-repo or template (e.g., {repo}-tier{tier}) and try that before universal.
• For auto, attempt remote repo_id-tier{tier} if local tier dir is missing.
• Optional fallback: download universal and slice locally (with clear warning), if enabled.

Acceptance Criteria

• Tiered checkpoints are automatically preferred when available.
• Clear behavior when tiered repos are missing (fallback or error).
• Documentation update for recommended tiered repo layout.