Avoid mixing BigInt and Number in marketplace purchaseAsset fee math

[Steve0x2a]

Summary

• Fixes TypeError Cannot mix BigInt and other types in MeshMarketplaceContract.purchaseAsset() when on-chain datum price ints deserialize as bigint.
• Introduces BigInt-safe computeMarketplacePurchasePayouts for owner fee and seller payout math (preserves prior ceil / 1 ADA minimum fee behavior).
• Adds regression tests (jest + zero-dep node:test wiring/legacy-throw checks).

Credits @rayan25nov for the clear repro and local BigInt fix suggestion.

Fixes #713
Closes #712

Test plan

• node --test packages/mesh-contract/src/marketplace/purchase-fee-math.node.test.mjs
• npx tsx import of computeMarketplacePurchasePayouts with BigInt price
• npm test / jest for @meshsdk/contract in full monorepo install

[Steve0x2a]

Closing in favor of existing open PRs that already address the same BigInt/Number mix in purchaseAsset (#714, #832). Thanks for the prior work — no need for a third patch on this path.