docs: add recipient address encoding guide for Solana-to-Base transfers

[faisalnugroho]

Summary

Fixes #190

The TokenLib.finalizeTransfer() function decodes EVM recipient addresses using address(bytes20(transfer.to)), which takes the first 20 bytes (left-aligned) of the bytes32 value. This is easy to misuse because common Solidity-style address-to-bytes32 encoding is often right-aligned.

Changes

Added a new section to base/README.md documenting:

• Correct encoding: bytes32(bytes20(evmAddress)) — left-aligned
• Incorrect encoding: bytes32(uint256(uint160(evmAddress))) — right-aligned (will decode wrong address)
• Quick reference table for all transfer directions
• JavaScript/TypeScript helper using viem's pad() function

[cb-heimdall]

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[faisalnugroho]

Hi! Following up on this docs PR. This PR documents the recipient address encoding requirements for Solana-to-Base bridge transfers, including the hex encoding format and common pitfalls.

All CI checks pass (StepSecurity ✅). Would love to get your review when you have a moment. Thanks!