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/Option.bux

@@ -1,6 +1,8 @@
 module Std::Option {
 import Std::Io::{PrintLine};
 
+extern func bux_exit(code: int);
+
 enum Option {
     Some(int),
     None,
@@ -39,4 +41,21 @@ func Option_UnwrapOr(o: Option, fallback: int) -> int {
     return fallback;
 }
 
+/* Unwrap Some or panic with a custom message */
+func Option_Expect(o: Option, msg: String) -> int {
+    if o.tag != Option_Some {
+        PrintLine(msg);
+        bux_exit(1);
+    }
+    return o.data.Some_0;
+}
+
+/* If o is Some return it, otherwise return other */
+func Option_Or(o: Option, other: Option) -> Option {
+    if o.tag == Option_Some {
+        return o;
+    }
+    return other;
+}
+
 }