feat: native multi-asset accounts (CreateAsset + multi-asset Transfer)

[SIDDHANTCOOKIE]

Summary

• Account balances move from a single native int to a per-asset dict ({"": amount, "<addr>.<ticker>": amount, ...}), with a per-account ticker registry (not global) to avoid namespace bloat/squatting.
• New CreateAsset(ticker, amount, recipient) transaction, inferred from a new optional ticker field (matching the existing implicit deploy/call inference style — no new tx_type field). Mints "<creator_address>.<ticker>" to recipient; the mint amount is not debited from the creator, only gas is.
• Plain Transfer gains an optional assets dict to move several assets atomically alongside the native amount.
• Gas/fees remain native-only, unchanged.
• Contract execution (msg.value, transfer_out) is intentionally left native-only in this PR — multi-asset support for contracts is scoped separately.
• Genesis loading, mc_getBalance RPC, and the CLI (balance, new create-asset command) updated for the new dict shape.

This is feature 1 of the "ychain" rework (per the linked Discord discussion with Bruno) and a prerequisite for the planned native DEX and UTXO/account hybrid work.

Test plan

• Updated all existing tests for the new balances/registry account shape.
• Added tests/test_multi_asset.py: CreateAsset mint/registry update, gas-only debit, duplicate-ticker rejection, malformed ticker (. in ticker), CreateAsset requires receiver/no data, multi-asset Transfer atomicity, insufficient-asset-balance rejection, state_root() determinism regardless of dict insertion order.
• Full suite: 238 passed, 2 deselected (pre-existing multiprocessing-timeout flakiness in test_contract_calls.py, confirmed present on unmodified main too).
• Manual smoke test: CreateAsset + multi-asset Transfer end-to-end via a script, main.py imports cleanly.

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