[codex] compare checkpoint inference runs

[salim4n]

Closes #49

What changed

• Loaded checkpoint payloads from native project storage when present and built Studio checkpoint views from native project directories.
• Linked checkpoint-backed inference runs back to their source checkpoint in the checkpoint panel.
• Labeled run comparison rows/options as training runs or inference replays so metrics are easier to interpret.
• Added coverage for selecting checkpoint-backed inference runs in comparison.

Validation

• bun test apps/studio/src/App.test.tsx
• bun run --cwd apps/studio typecheck
• cargo test --manifest-path apps/studio/src-tauri/Cargo.toml
• bun run ci