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.

src/lexer.bux

@@ -277,6 +277,7 @@ func lexKeywordKind(text: String) -> int {
     if String_Eq(text, "switch") { return tkSwitch; }
     if String_Eq(text, "case") { return tkCase; }
     if String_Eq(text, "default") { return tkDefault; }
+    if String_Eq(text, "unsafe") { return tkUnsafe; }
     if String_Eq(text, "async") { return tkAsync; }
     if String_Eq(text, "await") { return tkAwait; }
     if String_Eq(text, "spawn") { return tkSpawn; }