Enable user_stats module for achievements and leaderboards

[Jasper-Bekkers]

Summary

• Enables the user_stats module which was previously commented out
• Adapts the module to use the dynamic library loading pattern established in this fork
• Adds missing callback struct definitions required for Steam API responses

Changes

src/lib.rs

• Uncommented mod user_stats and pub use crate::user_stats::*
• Added CString import for string handling
• Added user_stats() method to Client implementation

src/user_stats.rs & src/user_stats/stats.rs

• Updated all Steam API calls to use dynamic library pattern (self.inner.lib.SteamAPI_* instead of sys::SteamAPI_*)
• For closures that need library access, cloned the lib Arc before capturing

steamworks-sys/src/lib.rs

• Added 8 callback struct definitions missing from auto-generated bindings:
  • UserStatsReceived_t - callback for RequestCurrentStats
  • UserStatsStored_t - callback for StoreStats
  • UserAchievementStored_t - callback for achievement storage
  • UserAchievementIconFetched_t - callback for achievement icon fetch
  • LeaderboardFindResult_t - callback for FindLeaderboard
  • LeaderboardScoreUploaded_t - callback for UploadLeaderboardScore
  • LeaderboardScoresDownloaded_t - callback for DownloadLeaderboardEntries
  • GlobalAchievementPercentagesReady_t - callback for RequestGlobalAchievementPercentages

Features Enabled

• Achievements: get/set/clear, display attributes, icons, global unlock percentages
• Leaderboards: find, create, upload scores, download entries, get metadata (name, entry count, sort method, display type)
• Stats: get/set i32 and f32 stats, store stats, reset all stats

Test plan

• cargo check passes successfully
• cargo fmt --all -- --check passes

🤖 Generated with Claude Code

[maxded]

Where do these bindings come from? How come they weren't generated to begin with?