MNT PART 1: core/types: adopt pyquarkchain account encoding

[ping-ke]

Summary

Adopt the pyquarkchain-compatible QuarkChain account representation and RLP codec for goshard's shard chain.

This PR establishes the account-level consensus format for the subsequent state, trie, snapshot, and command migrations.

Changes

• Replace the Ethereum StateAccount balance field with QuarkChain unified token balances.
• Add FullShardKey to the account consensus representation.
• Add pyquarkchain-compatible six-field account RLP encoding and decoding.
• Add QKC balance helpers and deep-copy support.
• Add account codec, balance, and copy tests.

Scope

This PR is limited to the account model and codec in core/types.

The following work will be included in subsequent PRs:

1. core/state

   • Integrate unified token balances into stateObject and StateDB.
   • Update state mutation, journaling, copying, and related tests.

2. trie, trie/bintrie, and triedb

   • Update account storage and trie commit/load logic.

3. core/state/snapshot and eth/protocols/snap

   • Update snapshot conversion and snap synchronization.

4. cmd/geth, cmd/evm, and remaining callers

   • Migrate account construction and replace direct StateAccount.Balance access with Balance().

This migration does not provide compatibility with existing Ethereum/geth data, old goshard data, or local pyquarkchain/goquarkchain database data. Local state only needs to remain internally self-consistent and can be rebuilt through P2P synchronization.

The current list-based token balance representation and 16 non-zero-token limit are intentional.

Tests

go test ./core/types

[syntrust]

$ go build ./...
# github.com/ethereum/go-ethereum/trie/bintrie
trie/bintrie/trie.go:222:2: cannot assign to acc.Balance (neither addressable nor a map index expression)
trie/bintrie/trie.go:248:30: acc.Balance.Bytes undefined (type func() *uint256.Int has no field or method Bytes)

[syntrust]

This comment is the only thing standing between the node and a silently wrong state root - nothing in the tree actually disables snap sync or the flat reader, and geth enables the snapshot by default. Once core/state is wired up in PR 2, core/state/database_mpt.go:159 calls EncodeMPTState() -> SlimAccountRLP on every commit, so the snapshot layer stores accounts with MNT balances and FullShardKey dropped, and core/state/reader.go:110 rebuilds StateAccount from them. That fails silently rather than loudly.

The conversion is also lossy in a second way the comment does not mention: FullAccount builds its balances with NewQKCTokenBalances(slim.Balance), which inserts an explicit zero entry when the balance is zero. Re-encoding then yields token_balances = 0x00c0 instead of the empty string, so even a pure-QKC account with a zero balance round-trips to a different leaf hash - snapshot-driven trie regeneration (core/state/snapshot/conversion.go) would produce a different state root.

SlimAccountRLP itself is on the unconditional commit path, so it cannot panic. Please put a real guard on the consuming side instead - refuse to construct the snapshot / flat reader, or return an error from the snapshot read path in core/state/reader.go - so that "must not be enabled" is enforced rather than documented.