Fix market order price rounding for builder-deployed perp dexs - #311

Problem

Exchange._slippage_price decides how many decimal places a market-order price may have:

asset = self.info.coin_to_asset[coin]
# spot assets start at 10000
is_spot = asset >= 10_000
...
return round(float(f"{px:.5g}"), (6 if not is_spot else 8) - self.info.asset_to_sz_decimals[asset])

Spot asset ids are index + 10000, but builder-deployed (HIP-3) perp dex asset ids are 110000 + i * 10000 + index (see Info.__init__). Those are also >= 10_000, so every HIP-3 perp is classified as spot and rounded to 8 - szDecimals decimals instead of 6 - szDecimals.

market_open / market_close on a HIP-3 dex can therefore build a price with more decimal places than perps allow, and the exchange rejects the order. For an asset with szDecimals == 0 and a mid of 0.0012345678, a 5% buy slippage price comes out as 0.0012963 (7 decimals) where the perp limit is 6.

#225 fixed the all_mids lookup for HIP-3 dexs but left this check untouched.

Fix

Spot asset ids live in [10000, 110000), so bound the check on both sides:

is_spot = 10_000 <= asset < 110_000

Tests

Adds tests/exchange_test.py covering _slippage_price for a perp, a spot pair, and a builder-deployed perp asset. The last one fails on master (0.0012963 != 0.001296) and passes with the fix. The existing suite still passes (39 tests).