feat: multi-instance closures, richer stdlib, and Rust-style diagnostics

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.

tests/error_golden/undeclared/src/Main.bux

@@ -0,0 +1,6 @@
+import Std::Io::PrintLine;
+
+func Main() -> int {
+    PrintLine(noSuchVar);
+    return 0;
+}