Implement Replay Recording and Playback Engine

[mac-dubem]

Description

Build a replay system that records every player action during a session as a timestamped event log and supports deterministic playback of that log. Replays let players review their own solutions, serve as proof-of-solve for on-chain achievement verification, and provide debugging data for the puzzle design team. Replay files are compact, portable, and verifiable.

Changes

• src/replay/mod.rs — New replay module with:

  • ReplayEvent enum: SessionStart, SessionEnd, HintReveal, AnswerAttempt { answer }
  • ReplaySession struct for recording, saving (.qreplay binary via bincode), loading, and playback
  • Microsecond-precision timestamps relative to session start
  • Puzzle content hash for integrity cross-checking on playback
  • Playback at configurable speed (1x, 2x, 4x) with inter-event timing
  • Replay summary (total time, hints used, attempt count, final score) printed at playback end

• src/main.rs — Added --playback <file> and --speed <1|2|4> CLI flags

• Cargo.toml — Added bincode dependency for binary serialization

• src/lib.rs — Registered pub mod replay

Tests

10 unit tests covering:

• Event recording order
• Monotonic timestamp generation
• bincode serialization roundtrip
• Save and load .qreplay files
• Integrity hash match/mismatch detection
• Empty replay playback (no panic)
• Default state verification
• Nonexistent and corrupted file error handling

All 175 tests pass (the 1 pre-existing failure in leaderboard::tests::replacement_when_full is unrelated).

Closes #47

[Mkalbani]

@mac-dubem please resolve conflict