Add int-specialized higher-order Iter helpers that accept fat function
pointers, plus an example using named funcs and capturing closures.
Fix selfhost C backend pointer field access and Array Push indexing.
Introduce fat function pointers (BuxFn {code, env}) so capturing closures
are heap-allocated per value in both bootstrap and selfhost. Expand
Array/Map/Set/String/Test/Result APIs, add proper tuple codegen and
error snippets with multi-char underlines, golden diagnostic tests, and
LSP diagnostics via buxc check.