test(models): Asset + Balance + Transaction (+14 tests)

[TaprootFreak]

Summary

Stage 26 of the coverage push. Pure-Dart model classes.

Model	Cases
`Asset`	5
`Balance`	5
`Transaction`	3
`TransactionTypes` enum	1

What each model pins

• Asset: `id` derived from `fastHash(chainId:address)`; `Asset.getId` static matches the instance getter; different chains / addresses produce different ids; `id` depends only on chain+address, not on metadata (`name` / `symbol` / `decimals` don't affect identity — important for repo de-duplication).
• Balance: `id` from `fastHash(wallet:chain:contract)`; `balance` amount is mutable; two Balances with the same identity tuple are `==` regardless of amount (pinned so a stream-emitted update with a new amount can still match an existing map key); different chain / wallet differs.
• Transaction: `isOutbound` true when sender matches wallet (EIP-55 normalised); false when sender differs; normalises hex-digit case via `fromHex` / `hexEip55` on both sides (NOT case-insensitive on the `0x` prefix — that breaks fromHex; the test pins the actual contract).
• TransactionTypes: values has exactly 5 entries (catches an accidental addition that would silently break the rendering switch).

Test plan

• `flutter analyze` clean
• `flutter test` — 14 / 14 passing locally
• CI green