Charge taker fee + maker rebate on fills

[ozpool]

What

Point-7 economics. Every fill recorded feeUsdc: "0" — the per-market takerFeeBps/makerRebateBps were defined but never applied. So no fees were collected and makers had no economic reason to quote.

How

On each fill, compute the fee/rebate as bps of notional, stamp them on the fill records, and settle into vault balances:

• Taker is debited takerFeeBps of notional (feeUsdc positive on the taker fill).
• Maker is credited makerRebateBps (feeUsdc negative = credit on the maker fill). The default schedule's rebate is negative (−2bps), so the maker is also debited — the engine handles the signed case.
• Vault balances clamp at zero. The net (taker_fee − maker_rebate) is the protocol's take.

Test

• New e2e fills_charge_fee_and_settle_vault: a matched fill now carries a non-zero, positive taker fee, and the taker's vault is debited by it.
• 34 lib + all integration tests green; fmt + clippy clean.

Notes

• No explicit protocol/insurance balance yet — the net take isn't credited anywhere, it just leaves the traders' vaults. That account lands with on-chain settlement.
• Recruiting external makers + designing an incentive/points program is business, out of code scope.