Summary

• Fix JSON serialization of felt types that are stored as struct value fields — they were
  being serialized as [u64, u64, u64, u64] Montgomery limbs instead of "0x..." hex strings
• Change MarshalJSON from pointer to value receiver on all felt types (Felt, Hash,
  TransactionHash, Address, ClassHash, SierraClassHash, CasmClassHash, StateRootHash)
• Add regression test covering all felt types through any interface (reproducing the jsonrpc
  server's serialization path)

Rationale

Go's encoding/json cannot call pointer-receiver methods on non-addressable values. The jsonrpc
server stores handler return values in response.Result any (server.go:519):

res.Result = tuple[0].Interface() // stores struct as value in `any`

Values extracted from any via reflection are not addressable. Since all felt MarshalJSON
methods used pointer receivers (*Felt, *Hash, etc.), encoding/json could not call them
and fell back to encoding the underlying [4]uint64 as a JSON array.

This violates the Starknet RPC spec
which defines FELT as "type": "string", "pattern": "^0x(0|[a-fA-F1-9]{1}[a-fA-F0-9]{0,62})$".

Observed behavior

{
  "transaction_hash": [10235509696732094000, 15042186987767562000, 2604752151520920600, 522667376275625800]
}

Expected behavior

{
  "transaction_hash": "0x8de..."
}

Timeline

The underlying issue (Felt.MarshalJSON using a pointer receiver) existed since the core/felt
package was created, but was latent because all response structs used *felt.Felt (pointer fields).
It only surfaced when v10 introduced value-type felt fields in handler return structs.

Changelog

Fixed

• starknet_addInvokeTransaction returning transaction_hash as an array of u64 limbs instead
  of a hex string, breaking starknet.js and other clients that expect the spec-compliant format
• All felt-derived types (Felt, Hash, Address, TransactionHash, ClassHash,
  SierraClassHash, CasmClassHash, StateRootHash) now serialize correctly as hex strings
  even when used as value fields in structs passed through any interfaces

Test plan

• New TestMarshalJSON_ValueInInterface reproduces the bug across all felt types
• go test ./core/felt/... — all pass
• go test ./rpc/... — all pass (v6 test updated to expect correct hex output)
• go test ./jsonrpc/... — all pass
• go test ./adapters/... — all pass