feat(compression): add compressMetadata and decompressMetadata helpers (#619)

[s6pa1rta3n-lab]

Summary of Changes

• Implements compressMetadata(obj: Record<string, unknown>, maxBytes = 512): string in src/compression.ts — serializes metadata to JSON and encodes as unpadded base64url.
• Implements decompressMetadata(encoded: string): Record<string, unknown> in src/compression.ts — decodes base64url and parses JSON into a plain object.
• Validates base64url character set and structure, throwing SdkError with SdkErrorCode.CONTRACT_REJECTED on invalid base64url characters, non-string/empty inputs, malformed JSON, or non-object payloads.
• Enforces configurable maxBytes size limit (default 512 bytes), throwing SdkError with SdkErrorCode.CONTRACT_REJECTED when the resulting payload exceeds the limit.
• Exports compressMetadata and decompressMetadata alongside existing compression utilities and types in src/index.ts.
• Adds extensive unit tests in test/metadataCompression.test.ts verifying round-trip deep equality, empty object handling, nested/unicode objects, custom and default maxBytes boundaries, and various invalid input conditions.

Acceptance Criteria Checklist

• compressMetadata(obj: Record<string, unknown>): string exported — serializes to JSON then encodes as base64url (no padding =)
• decompressMetadata(encoded: string): Record<string, unknown> exported — decodes base64url and parses JSON; throws SdkError with CONTRACT_REJECTED code on invalid input
• compressMetadata throws SdkError with CONTRACT_REJECTED code if the resulting base64 string exceeds maxBytes (default 512 bytes, configurable via second argument)
• Round-trip: decompressMetadata(compressMetadata(obj)) deep-equals obj for any JSON-serialisable input
• Both exported from src/index.ts
• Unit tests: round-trip correctness, oversized payload throws, invalid base64 input throws, empty object round-trips

Closes #619
/claim #619

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC