feat: implement fractional fee calculation and treasury routing

[thegreatfeez]

Description

Implements protocol fee skimming during release_funds. When a treasury and a non-zero fee rate are configured (via set_fee_config), the release now splits the escrowed amount into amount - fee to the seller and fee to the treasury, emitting a ProtocolFeeCollected event atomically alongside FundsReleased. If no fee is configured, or the fee truncates to 0 for a given amount, the seller receives 100% of the funds and only FundsReleased fires — matching pre-existing behavior exactly.

The fee is computed with u128 for the intermediate multiplication (guarded with checked_mul) to avoid overflow, and integer division truncates the fee downward, so rounding always favors the seller.

Depends on #58 (this branch is built on top of feat/fee-storage, since fee skimming needs the DataKey::FeeRate/DataKey::Treasury storage and read_fee_rate/read_treasury helpers introduced there). This PR cannot merge before #58/PR #90 merges — the diff below currently includes that PR's commit as well; it will shrink to just this issue's changes once #90 lands and this branch is rebased onto main.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

1. cargo test — all existing tests plus 5 new tests pass (107 total): fee-uninitialized defaults to a full payout, a configured fee correctly splits seller/treasury balances and emits both events, fee rounding truncates in the seller's favor on an odd amount, and a tiny amount whose fee truncates to 0 still pays the seller 100% with no fee event
2. cargo fmt --all -- --check — clean
3. cargo clippy --all-targets --all-features -- -D warnings — clean
4. cargo build --target wasm32v1-none --release — builds successfully

Closes #59

Checklist

• Code follows style guidelines (cargo fmt run)
• Tests added/updated and cargo test passes
• Documentation updated