fix(contract): safe BigInt arithmetic in MeshMarketplaceContract.purchaseAsset

[OscarOzaine]

Summary

This is a suggested improvement follow-up to #714 (which fixed the TypeError: Cannot mix BigInt and other types crash in purchaseAsset).

While #714's Number() conversion resolves the crash, it introduces silent precision loss for NFT prices above Number.MAX_SAFE_INTEGER (~9,007,199 ADA) and leaves several other issues in the same function unaddressed. This PR addresses them all.

Changes

purchaseAsset — BigInt-safe arithmetic (fixes #714's root cause)

// Before (PR #714): Number() conversion loses precision above ~9M ADA
let ownerToReceiveLovelace =
  (Number(inputDatum.fields[1].int) * this.feePercentageBasisPoint) / 10000;

// After: all arithmetic stays in BigInt; convert to string only at txOut boundary
const priceBig = BigInt(inputDatum.fields[1].int);
const feeBig = BigInt(Math.round(this.feePercentageBasisPoint));
let ownerToReceiveLovelace = (priceBig * feeBig + 9999n) / 10000n; // ceiling division

• (a * b + 9999n) / 10000n is exact BigInt ceiling division — equivalent to Math.ceil without any float arithmetic.
• Seller payment: priceBig + BigInt(lovelaceEntry.quantity) — no IEEE-754 rounding regardless of price magnitude.

Free-listing spurious fee fix

Previously, a listing with price = 0 would hit the 1-ADA minimum floor and charge the buyer 1 ADA to the marketplace owner, even though the on-chain validator requires value_geq(0) (no floor). Added priceBig > 0n guard.

lovelaceEntry — descriptive error instead of opaque crash

// Before: throws "TypeError: Cannot read properties of undefined (reading 'quantity')"
const inputLovelace = marketplaceUtxo.output.amount.find((a) => a.unit === "lovelace")!.quantity;

// After: throws a meaningful domain error
const lovelaceEntry = marketplaceUtxo.output.amount.find((a) => a.unit === "lovelace");
if (!lovelaceEntry) throw new Error("purchaseAsset: marketplace UTxO is missing a lovelace entry");

Constructor — feePercentageBasisPoint range validation

A value > 10000 (> 100%) silently produces ownerToReceiveLovelace > price, making the transaction unbalanceable. Now throws early with a clear message.

Issues addressed

#	Severity	Issue
1	Medium	Number(bigint) precision loss for prices > ~9M ADA → wrong fee and seller amounts
2	Medium	Float addition off-by-1 at MAX_SAFE_INTEGER boundary in seller payment
3	Medium	Free listing (price=0) triggers spurious 1-ADA mandatory fee
4	Medium	Non-null assertion crash with opaque error on UTxOs missing lovelace
5	Medium	feePercentageBasisPoint > 10000 accepted silently

The shared this.mesh builder TOCTOU (concurrent calls swapping tx hexes) is a broader architectural issue across all contract methods and is left for a separate discussion.

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