feat: Add emergency_withdraw to farming-pool contract

[simplicityf]

Related Issue

Closes #24

Problem

The existing pause mechanism prevents new operations when a critical vulnerability is discovered, but leaves staked and locked tokens permanently inaccessible. There was no admin path to return funds to users during an emergency.

Solution

Introduces emergency_withdraw, an admin-only function callable exclusively while the pool is paused. It returns all tokens held for a given user (both the lock position and the stake balance), preserves any accrued credits in a new BankedCredits storage key for a future claim mechanism, and emits a fully auditable emrg_exit event.

Changes

types.rs

• Added PoolError — a #[contracterror] enum with NotPaused = 13 and NoActiveStake = 14
• Added DataKey::BankedCredits(Address) — persistent storage key for credits preserved after an emergency withdrawal

lib.rs

• emergency_withdraw(env, user) -> Result<i128, PoolError> — transfers both the lock position amount and the stake amount back to the user, writes any banked credits to BankedCredits storage before removing the records, and emits ("pool", "emrg_exit") with (admin, user, total_returned)
• get_banked_credits(env, user) -> i128 — public query for credits saved by a prior emergency withdrawal; returns 0 if none
• set_banked_credits — private helper that writes to BankedCredits with TTL bump

test.rs

• test_emergency_withdraw_while_paused — locks 600 tokens, stakes 400, triggers credit checkpoints (8 000 credits total), pauses, calls emergency_withdraw, asserts all 1 000 tokens returned, position and stake records cleared, and 8 000 credits preserved in get_banked_credits
• test_emergency_withdraw_while_unpaused_returns_not_paused — asserts Err(PoolError::NotPaused) when called on a live pool

Safeguards

Safeguard	How it's enforced
Admin-only	get_admin(&env).require_auth() at function entry
Paused-only	Returns Err(PoolError::NotPaused) if !pool_is_paused — prevents routine extraction of user funds
Credits preserved	Banked credits are written to BankedCredits before position/stake records are removed
Fully auditable	emrg_exit event emitted for every withdrawal with (admin, user, amount)

Test results