feat: add stdlib module support with auto-import resolution

- Parse module paths in stdlib (module Std::Array { ... })
- Collect stdlib declarations and inject into user modules before sema
- Add dkExternFunc support throughout pipeline (parser, sema, hir, c backend)
- Auto-dereference pointer types for field access (arr->field)
- Add hArrowField HIR node for cleaner C emission
- Fix resolveTypeExpr for pointer and cast types
- Fix struct field lowering to use resolveTypeExpr
- Allow int <-> uint implicit conversion for bootstrap convenience
- Add stdlib/Std/Array.bux with dynamic array primitives
- Add stdlib_test demonstrating Array usage via import Std::Array::{Array};

src/types.nim

@@ -120,6 +120,9 @@ proc isAssignableTo*(a, b: Type): bool =
   # smaller int -> int/uint
   if b.kind == tkInt and a.kind in {tkInt8, tkInt16, tkInt32}: return true
   if b.kind == tkUInt and a.kind in {tkUInt8, tkUInt16, tkUInt32}: return true
+  # int <-> uint (for convenience in bootstrap)
+  if a.kind == tkInt and b.kind == tkUInt: return true
+  if a.kind == tkUInt and b.kind == tkInt: return true
   # numeric exact match required otherwise
   if a.isNumeric and b.isNumeric: return false
   # bool across widths