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.
- Add String_IsNull and String_Offset wrappers in lib/String.bux
- Add bux_str_is_null and bux_str_offset runtime functions in rt/runtime.c
- Update String_Replace to use String_IsNull instead of String_Len(pos)==0
- Replace pointer-to-uint casts (as uint == 0, pointer arithmetic) across apps/
- Parser newline skipping fixes in src/parser.bux
dimgigov