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/c_backend.nim

@@ -195,6 +195,10 @@ proc emitExpr(be: var CBackend, node: HirNode): string =
     let base = be.emitExpr(node.fieldPtrBase)
     return &"(&({base}.{node.fieldName}))"
 
+  of hArrowField:
+    let base = be.emitExpr(node.arrowFieldBase)
+    return &"(&({base}->{node.arrowFieldName}))"
+
   of hIndexPtr:
     let base = be.emitExpr(node.indexPtrBase)
     let idx = be.emitExpr(node.indexPtrIndex)