Writergate: Zig I/O Interface Overhaul

Zig's I/O interface underwent a major overhaul called "Writergate" (late 2023–August 2025), replacing generic types with concrete types using vtables and explicit buffering, moving from `std.io` to `std.Io`. The change eliminates the "poisoning" problem where generic writer/reader parameters forced entire codebases to become generic, improving compile times and API reusability while enabling async support and better error handling through a three-tier vtable architecture. The new design treats I/O like memory allocation, with callers providing buffers and requiring explicit flush calls.