Async Io Model

If you're working in Turso's core codebase, this is the pattern guide you'll need whenever touching I/O. It covers their cooperative async model built on explicit state machines and IOResult enums instead of Rust's async/await. The real value is in the re-entrancy pitfall section, which shows exactly how mutating state before a yield point causes subtle bugs when functions get called again after I/O completes. You get concrete examples of the state machine pattern, CompletionGroup for waiting on multiple operations, and the helper macros like return_if_io! that make it less painful to write. This is reference material for a specific codebase architecture, not a general async patterns guide.