Improve process_slots logger parameter design and consider CachedState architecture

The `process_slots` function currently takes `logger: anytype` as a parameter, which feels awkward and reduces readability.

```zig
pub fn process_slots(self: *BeamState, allocator: Allocator, target_slot: Slot, logger: anytype) !void
```

## Proposed Solutions

### CachedState wrapper
- Create `CachedState` struct with `BeamState` and logger as properties
- Better performance for block processing
- Efficient justifications handling (flatten only when needed for SSZ ops)

## Context
here https://github.com/blockblaz/zeam/pull/260#discussion_r2402001864
https://github.com/blockblaz/zeam/pull/260#discussion_r2402253194