feat: add reputation system with staking mechanics

[Primex-hub]

Summary

Implements the full reputation system with staking mechanics for the Hello-world idea-sharing platform, as specified in Issue #102.

Changes

src/lib.rs — New Soroban smart contract

Staking:

• stake_tokens(user, amount) — lock tokens to participate; requires amount > 0
• unstake_tokens(user, amount) — withdraw staked tokens (blocked if slashed)
• get_staked_balance(user) — query staked balance

Ideas:

• create_idea(author, title, content, tags) — post an idea (requires staked tokens, awards +10 reputation)
• get_idea(idea_id) — retrieve idea data

Voting:

• vote_idea(idea_id, voter, is_upvote) — upvote/downvote with reputation-weighted voting
  • 0-49 rep: weight 1
  • 50-99 rep: weight 2
  • 100+ rep: weight 3
• Authors receive bonus reputation when receiving upvotes from high-rep users

Reputation & Leaderboard:

• get_reputation(user) / get_user_data(user) — query user stats
• refresh_leaderboard(users) — rebuild leaderboard snapshot (selection sort by rep descending)
• get_leaderboard(limit) — return top-N users by reputation

Slashing:

• slash_user(caller, target, reason) — admin-only; halves reputation, flags user to prevent unstaking
• get_slash_count() — audit trail count

Admin:

• set_admin(admin) — one-time admin bootstrap

Cargo.toml

• Added [dev-dependencies] with soroban-sdk testutils feature

Tests (19 test cases)

• Staking: accumulate, zero-panic, unstake, insufficient-balance panic
• Ideas: create, create-without-stake panic, multiple ideas
• Voting: basic upvote, high-rep weighted upvote, weight-2 upvote, downvote
• Slashing: admin slash, non-admin panic, slashed-user-cannot-unstake
• Leaderboard: ordering, limit
• Full lifecycle integration test

Closes #102