fix: support osaka compilation with Solidity 0.8.31

[sanbir]

Summary

This updates the playground Solidity compiler to a version that supports Osaka-compatible contracts requiring Solidity ^0.8.31.

Changes

• util/compiler.ts
• Set default compiler to soljson-v0.8.31+commit.fd3a2265
• Keep EOF mapping behavior (EOF -> prague) while preserving normal hardfork passthrough

Why

fork=osaka compilation failed for newer contracts and Osaka-specific assembly paths when the playground was pinned to 0.8.27.

Closes #409

[vercel]

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[cursor]

PR Summary

Low Risk
Single constant/version bump; behavior changes are limited to using a newer solc binary and may affect compilation outputs for some contracts.

Overview
Updates the pinned Solidity compiler version in util/compiler.ts from 0.8.27 to 0.8.31 (Osaka-compatible), so contracts requiring ^0.8.31 compile successfully.

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