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/hir.nim

@@ -22,6 +22,7 @@ type
     hLoad
     hStore
     hFieldPtr
+    hArrowField
     hIndexPtr
     # Functions
     hCall
@@ -84,6 +85,9 @@ type
     of hFieldPtr:
       fieldPtrBase*: HirNode
       fieldName*: string
+    of hArrowField:
+      arrowFieldBase*: HirNode
+      arrowFieldName*: string
     of hIndexPtr:
       indexPtrBase*: HirNode
       indexPtrIndex*: HirNode