feat: implement concrete collective memory layer and protocol tests

Description

The pitch.

### Summary
Implements a concrete `CollectiveMemory` layer class and adds corresponding unit tests to verify proper integration, layout structure, and protocol validation.

### Changes
* **Core Implementation**: Created `collective_memory.py` inside `nest_core/layers` to provide a robust implementation of the collective memory protocol.
* **Test Suite Expansion**: Added `test_collective_memory.py` to cover runtime protocol compliance check validation and custom functionality.
* **Code Quality**: All new components have been verified locally; formatting and linting pass with `ruff`.

### Verification Status
- [x] `uv run ruff check .` passed completely.
- [x] `uv run ruff format --check .` passed cleanly.
- [x] Targeted `pyright` checks on new files verified type safety.