Add an optional mint authority to fungible tokens for controlled supply:
create with a designated minter, mint additional supply, rotate the
authority to a new key, or permanently revoke it to fix the supply.
The authority is stored inline on `TokenDefinition::Fungible` as
`authority: Option<AccountId>` (`Some(id)` = mintable by `id`, `None` =
fixed supply). Keeping it a plain `Option<AccountId>` rather than a custom
wrapper type leaves account state decodable by `spel inspect`; the
require/rotate/revoke guard logic lives inline in the handlers.
LEZ rejects a transaction that lists the same account id twice, so one
instruction cannot statically express both "the definition account is the
authority and signs" (self/PDA authority) and "a distinct rotated account
signs" (external authority) — they need opposite signer markers. Each
privileged operation is therefore split into a self and an external
variant:
- `Mint` / `SetAuthority` — the definition account is the signer.
- `MintWithAuthority` / `SetAuthorityWithAuthority` — a distinct authority
account is the signer; the definition account does not sign.
Creation via `NewFungibleDefinition { mint_authority, .. }`; an all-zero
authority id is rejected. The AMM's LP token uses self/PDA authority — its
stored authority is the LP definition PDA, minted only by the pool via
chained calls.
Covered by token unit tests and zkVM integration tests: creation with and
without an authority, self- and external-authority mint, rotation, and
external rotate/revoke. IDLs regenerated.
