Add partial_position_freeze() locking only a portion of a staking position

[Rickyy1017]

Summary

Currently freeze (Issue #71) applies to an entire position. Add a partial freeze mechanism where the admin can freeze a specific token amount within a position — the frozen portion cannot be unstaked while the unfrozen portion remains freely accessible.

Acceptance Criteria

• FrozenAmount(Address) persistent storage: i128 amount frozen within user's position
• partial_freeze(admin, user: Address, amount: i128) — freezes amount of user's position
• partial_unfreeze(admin, user: Address, amount: i128) — releases frozen amount
• get_frozen_amount(user: Address) -> i128 read-only query
• get_available_amount(user: Address) -> i128 = position.amount - frozen_amount
• unstake only allows withdrawal up to available_amount — revert with AmountFrozen if exceeding available
• partial_frozen event: (user, frozen_amount, total_position, ledger)
• Cannot freeze more than current position.amount — revert with FreezeExceedsPosition
• Tests: partial freeze limits unstake, unfreeze restores access, freeze more than position rejected, rewards still accrue on frozen amount

Notes

• Distinct from Issue Add total_rewards_paid running counter to track lifetime payouts #71 (full position freeze flag) — this tracks a specific frozen token amount.
• Rewards accrue on the full position including frozen portion — only unstaking is restricted.