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.

bootstrap/source_location.nim

@@ -3,6 +3,10 @@ type
     line*: uint32      ## 1-based
     column*: uint32    ## 1-based (UTF-8 byte offset in line)
     offset*: uint32    ## byte offset from start of file
+    file*: string      ## source file path (empty if unknown)
 
 proc `$`*(loc: SourceLocation): string =
-  $loc.line & ":" & $loc.column
+  if loc.file.len > 0:
+    loc.file & ":" & $loc.line & ":" & $loc.column
+  else:
+    $loc.line & ":" & $loc.column