Implement Session Module

[Mkalbani]

Description

Create a session module that manages a single play session, tying together player, puzzle progress, timer, and score.

Acceptance Criteria

• A session module exists under src/session/mod.rs
• Session struct holds player ref, active puzzle, timer, and current score
• Session can be started, paused, and ended
• On end, session data is passed to persistence and leaderboard modules
• Unit tests cover session lifecycle and data handoff