feat(staking): implement early withdrawal penalty

[munyanezaarmel]

Summary

Implements early withdrawal penalty for staking as requested in #242.

Behaviour

• Withdrawing before lock expiry deducts a penalty (default 10%)
• LockPeriod::Flexible stakes are never penalised
• Penalty is added to the reward pool, benefiting remaining stakers
• Admin can adjust the rate between 0–50% via set_early_withdrawal_penalty()

Changes

• contracts/staking/src/lib.rs — updated unstake(), added admin setter/getter, new EarlyWithdrawal event, new storage field
• contracts/staking/src/errors.rs — added EarlyWithdrawalPenaltyApplied variant
• propchain-traits/src/constants.rs — added DEFAULT_EARLY_WITHDRAWAL_PENALTY_BPS, MAX_EARLY_WITHDRAWAL_PENALTY_BPS

Tests

5 new #[ink::test] cases covering: penalty applied, flexible exempt, no penalty after expiry, admin-only setter, cap enforcement.

Closes #242

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[nanaf6203-bit]

LGTM