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.

lib/Os.bux

@@ -6,6 +6,7 @@ extern func bux_getenv(name: String) -> String;
 extern func bux_setenv(name: String, value: String) -> int;
 extern func bux_getcwd() -> String;
 extern func bux_chdir(path: String) -> int;
+extern func bux_exit(code: int);
 
 func Os_ArgsCount() -> int {
     return bux_argc();
@@ -31,4 +32,9 @@ func Os_Chdir(path: String) -> bool {
     return bux_chdir(path) == 0;
 }
 
+/* Terminate the process with the given exit code */
+func Os_Exit(code: int) {
+    bux_exit(code);
+}
+
 }