Anchor Constraints

Declarative validation rules on Anchor account fields. Key constraints: `#[account(mut)]` (writable), `#[account(init, payer=x, space=n)]` (create), `#[account(seeds=[...], bump)]` (PDA validation), `#[account(has_one=field)]` (field equality), `#[account(constraint = expr)]` (custom boolean), `#[account(close=target)]` (close and reclaim rent).