Sui Address Balance support and coin-model refactor

[0xh3rman]

Support Sui token transfers and swaps where the token amount lives in the per-address Balance accumulator. Gas payment still comes from a regular Coin<SUI> object owned by the sender; sponsored / fully gasless transactions are tracked separately in #1164.

Changes:

• SuiClient::get_coins joins ListOwnedObjects with GetBalance and returns OwnedCoins<Coin> { coin_type, coins, address_balance }. Spend builders route through 0x2::coin::redeem_funds<T> when Address Balance covers the amount; mixed token sources use coin objects as the base and withdraw only the shortfall.
• Merge SuiCoin + CoinAsset + Coin into a single Coin model with strong-typed Object { object_id: Address, digest: Digest, version: u64 }. Removes the From<SuiCoin> for Coin / TryFrom<SuiCoin> for CoinAsset converters and get_coin_assets_by_type.
• Native SUI / staking: support pure-accumulator and pure-coin paths. Hybrid is rejected early because gas budget is unknown at build time. Pure-accumulator paths still require at least one Coin<SUI> for gas; missing gas coins are rejected with "No SUI coins available for gas".

Verification: gem_sui 63 tests pass (--all-features, includes live mainnet integration); swapper 230 tests pass. New tests cover pure-withdraw, mixed shortfall-only withdrawal (asserts the withdrawal amount is exactly amount - coin_total), missing-gas-coin rejection, and hybrid-native rejection.

[gemini-code-assist]

Code Review

This pull request refactors the Sui integration to introduce the OwnedCoins struct, which encapsulates both individual coin objects and address-level balances. It updates balance validation, transfer builders, and staking logic to utilize this new structure. The review feedback highlights critical correctness bugs in the staking and transfer builders when the required amount is partially covered by the address balance and partially by coin objects, as well as a potential overflow panic in the coin_total calculation.