feat(lp-reward): implement liquidity provider reward contract for key pairs

[CathyZaks]

Header

Liquidity Provider Reward Contract for Key Pairs

Problem

Liquidity providers on key pairs currently lack a dedicated reward mechanism to lock tokens as liquidity, earn a proportional share of trading fees generated by pair trading activity, and claim accumulated fee rewards or withdraw liquidity with their accrued yield.

Solution

Implemented a dedicated LpRewardContract crate (and integrated equivalent entrypoints into CreatorKeysContract) using high-precision accumulator accounting (SCALE = 1e12) and basis-point proportional share calculations (BPS_SCALE = 10_000):

• add_liquidity(key_id, amount) / add_liquidity_for(caller, key_id, amount): Locks tokens, calculates proportional pool share in basis points, records the position, and emits an LPAdded event.
• remove_liquidity(lp_id): Computes accrued fee rewards plus principal, updates pool totals, clears the position, and emits an LPRemoved event.
• claim_lp_rewards(lp_id): Claims pending accrued fee rewards without removing principal liquidity and emits an LPRewardClaimed event.
• get_lp_position(lp_id): Returns contribution amount, basis-point pool share, and current pending reward amount.
• accrue_trading_fee(key_id, fee_amount): Accrues trading volume fees into the key pool reward accumulator for proportional distribution across all active liquidity providers.

Changes

• lp-reward-contract/Cargo.toml: Created workspace member crate with Soroban SDK dependencies and test utilities.
• lp-reward-contract/src/lib.rs: Core LpRewardContract implementation with data structures, error handling, events, and accumulator fee distribution.
• lp-reward-contract/src/test.rs: Unit test suite covering proportional shares, fee accrual over trading volume, reward claiming, liquidity removal, event emission, and parameter validation.
• Cargo.toml: Added "lp-reward-contract" to workspace members.
• creator-keys/src/lp_reward.rs: Matching module for LP reward tracking in the core creator-keys contract.
• creator-keys/src/lib.rs: Exposed LP reward entrypoints on CreatorKeysContract.
• creator-keys/src/test_lp_reward.rs: Integration tests verifying client invocations and fee distributions on CreatorKeysContract.
• .gitignore: Added general pattern for **/test_snapshots/.

Testing

• cargo fmt --all -- --check: Passed
• cargo clippy --workspace --all-targets -- -D warnings: Passed (0 warnings)
• cargo build --workspace: Passed
• cargo test -p lp-reward-contract: 7 passed, 0 failed
• cargo test -p creator-keys --lib test_lp_reward: 4 passed, 0 failed

Closes #1002

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