Implement Session Layer

[piraz]

This issue tracks the implementation of the Session Layer (Unit of Work) for sqlok.

Roadmap:

• Identity Map: map[Type][ID] -> *obj to manage object instances.
• Conflict Strategy: Return error on identity conflict (V1).
• Session Lifecycle: Methods for Add, Load, Flush, and Commit.
• State Tracking (Dirty Checking): Lightweight FNV-1a hashes for field-level comparison.

Note: Core Mapper implementation and Refactoring are considered separate scopes.