feat: add protocol fee for escrow releases — platform takes a small percentage of each release

[BigBen-7]

Description

A configurable protocol fee deducted from each escrow release and sent to a treasury address funds platform operations sustainably.

What to do

• Add DataKey::ProtocolFeeBps and DataKey::ProtocolTreasury to storage_types.rs
• Add set_protocol_fee(e, admin, fee_bps: u32, treasury: Address) — admin-only, fee_bps <= 500 (max 5%)
• In release_escrow: compute and deduct fee, transfer to treasury, release remainder to beneficiary
• Add get_protocol_fee(e) -> (u32, Option<Address>) view
• Add tests: 0% fee, 1% fee correctly deducted, >500bps panics

Files touched

• src/storage_types.rs
• src/escrow.rs
• src/contract.rs
• src/escrow_test.rs

PR requirement

• Closes #[issue_id] — Timeframe: 72 hours