fix(stablecoin-exchange): calculate flip order amount from actual received funds

[fgimenez]

Closes CHAIN-387

Fixes flip order issues for both ask->bid and bid->ask scenarios.

• ask->bid
  When an ask flip order is filled, the maker receives quote tokens rounded down. The flip logic then tried to place a bid using the original base amount, which could require more quote for escrow than was actually received.

With the changes in this PR we apply consistent rounding when calculating the flip amount:

let quote_received = base_to_quote(fill_amount, order.tick(), RoundingDirection::Down);
let flip_amount = quote_to_base(quote_received, order.flip_tick(), RoundingDirection::Down);

By rounding down consistently, the flip order is sized to what can actually be escrowed with received funds.

• bid -> ask

When a bid flip order is partially filled and the maker withdraws their internal balance, the subsequent fill would still create a flip order by using tokens just credited from the current fill. This violated the design intent that flip orders should only use pre-existing internal balance.

In this case we use order.amount() instead of fill_amount:

let flip_amount = if order.is_bid() {
    order.amount()  // Use original amount, not fill_amount
} else {
    // ask->bid rounding-aware calculation
};

This ensures the flip requires the original order amount for escrow, which won't be available if the maker withdrew after partial fills.

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1 Skipped Deployment

Project	Deployment	Review	Updated (UTC)
tempo-docs	Ignored	Preview	Dec 31, 2025 4:50pm

[0xKitsune]

IIUC, this implies that the flip order will (very) slowly decrease in quantity as it flips? It might be useful to document this.

[fgimenez]

yep done here 7abcf27

[fgimenez]

closing in favor of #1824