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/bux.toml

@@ -0,0 +1,7 @@
+[Package]
+Name    = "undeclared"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"

tests/error_golden/undeclared/expected.err

@@ -0,0 +1,7 @@
+error: type errors in project
+error: undeclared identifier 'noSuchVar'
+  --> FILE:4:15
+    |
+ 4 |     PrintLine(noSuchVar);
+    |               ^^^^^^^^^
+   = help: check the spelling, or import the symbol from the right module

tests/error_golden/undeclared/src/Main.bux

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