feat: implement concrete collective memory layer and protocol tests

[Arjun10348]

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

• uv run ruff check . passed completely.
• uv run ruff format --check . passed cleanly.
• Targeted pyright checks on new files verified type safety.