Add MatFormer manifest for tier-sliced checkpoints with auto-fetch and strict validation

[plugyawn]

Problem

We need a reliable way to auto-fetch tier-sliced MatFormer checkpoints (esp. HF repos) and fail cleanly when slices are missing. Current load logic only checks local "-tierN" folders; hub downloads always pull the universal checkpoint.

Proposed solution

• Introduce matformer_manifest.json (schema v1) with common_files and per-tier files (relative to the manifest) plus optional metadata (tier, base/intermediate sizes).
• Update scripts/export_matformer_tiers.py to write the manifest alongside the universal checkpoint, listing tier file paths + common files (tokenizer, config, etc.).
• Add a hub download helper that can fetch only a specific file list (used for manifest + tier files).
• In client init, when --matformer-load-strategy auto|sliced and tier > 0, attempt to use the manifest to fetch a tier slice; if missing or incomplete, fail cleanly in sliced, and fallback to universal in auto.
• Ensure config validation uses matformer_tier + matformer_base_intermediate_size from sliced checkpoints.

Error cases

• Missing manifest when sliced is requested → error with clear remediation.
• Manifest present but tier missing → error (sliced) or fallback (auto).
• Manifest lists files missing from repo/local path → error with missing file list.

Tests

• Unit tests for manifest parsing and tier file selection.
• Tests for missing-tier + sliced behavior.
• Run cargo test -p psyche-client (and psyche-modeling if needed).

Notes

• HF repo layout will allow tier files in subdirs; manifest paths are relative to the manifest.
• Keep backward compatibility with local -tierN directories when manifest is absent.