Remove the `active: bool` field from `PoolDefinition` and replace it with an
implicit invariant: a pool is considered active when
`liquidity_pool_supply >= MINIMUM_LIQUIDITY`.
BREAKING CHANGE: `PoolDefinition` Borsh serialization format has changed.
Existing on-chain pool accounts encoded with the `active` field are
incompatible with this version.
Closes#25
Adds a new `SyncReserves` instruction that updates a pool's recorded
reserves to match the actual vault balances. This allows the pool to
absorb donations (direct token transfers to vaults) without breaking
the invariant — only upward adjustments are permitted; vaults may
not be under-collateralized relative to reserves.
Vault reading helpers (`read_fungible_holding`,
`read_vault_fungible_balances`) are implemented in `amm_core` so they
can be shared across instructions without crossing crate boundaries.
