feat(stats): add bounded on-chain leaderboard queries

[Oyinkans0la12]

Closes #194

Summary

Add bounded on-chain leaderboard APIs for wins and best streak with deterministic pagination.

Changes

• Added leaderboard entry type
• Added bounded leaderboard index maintenance
• Added paginated leaderboard query endpoints
• Implemented deterministic tie-breaking
• Added test coverage for pagination, ordering, and consistency

Ordering

Primary sort:

• Wins (or Best Streak)

Tie-breaking:

• Deterministic using existing stable project identifiers

This guarantees stable pagination across repeated queries.

Storage

The leaderboard index is maintained as a bounded collection to prevent unbounded contract storage growth.

Testing

Covered:

• Wins leaderboard
• Best streak leaderboard
• Multi-page pagination
• Stable tie-breaking
• Leaderboard updates after stat changes
• Bounded storage behavior
• Consistency with per-user statistics

Validation

Executed:

cargo test -p <affected-crate>
cargo clippy --workspace --all-targets -- -D warnings
cargo build --target wasm32-unknown-unknown --release

Scope

This PR intentionally focuses only on bounded leaderboard indexing and paginated read APIs. It does not redesign the statistics system, storage architecture, ranking engine, or introduce off-chain indexing.

[josephchimebuka]

@Oyinkans0la12 Please fix conflicts

[Oyinkans0la12]

@Oyinkans0la12 Please fix conflicts

Resolved already, thanks!

[Oyinkans0la12]

Conflicts resolved, please merge.