feat(crdt): add PN-Counter, OR-Set, and LWW-Map CRDTs

[brunoga]

This PR introduces three new state-based Conflict-free Replicated Data Types (CRDTs) to the crdt package. These types provide high-level
abstractions for common distributed data patterns, all leveraging the project's core CRDT[T] generic base and Hybrid Logical Clocks (HLC).

New CRDTs

• PN-Counter: A Positive-Negative Counter that supports both increments and decrements. It tracks independent totals across nodes and resolves
  to a single convergent value (sum(increments) - sum(decrements)).
• OR-Set (Observed-Remove Set): An "Add-Wins" set implementation. It allows concurrent additions and removals of the same element; in the event
  of a conflict, the addition takes precedence.
• LWW-Map (Last-Write-Wins Map): A generic key-value map where concurrent writes to the same key are resolved using HLC timestamps. It supports
  both Set and Delete operations with proper tombstone propagation.

Key Changes & Refactoring

• Core Integration: While Set and Map were initially drafted with standalone logic, they have been refactored into thin wrappers around the
  CRDT[T] base. This ensures consistent behavior for synchronization, delta application, and state-based merging across all types.
• Code Simplification: By delegating concurrency control and causal tracking to the underlying CRDT[T] engine, the implementation of individual
  types is now significantly cleaner and less prone to manual locking or timestamp errors.
• Convergent Testing: Each new type includes a comprehensive test suite (counter_test.go, set_test.go, map_test.go) verifying:
  • Basic CRUD operations.
  • Idempotency: Multiple merges of the same state produce no side effects.
  • Commutativity: The order of merges does not affect the final state.
  • Convergence: Nodes with different edit histories reach the same state after synchronization.