fix(transform): preserve export keywords on nested namespaces (#360)

Fixes #359

Removes two problematic code sections from PR #352:

1. Lines 26-28: Unnecessary duplicateExports() call that caused
   superfluous export name duplication in namespace bodies

2. Lines 596-605: Incorrect removal of export keywords from nested
   namespaces, which changed their accessibility from public to private

The plugin should bundle declarations, not transform their semantics.
Nested namespaces with export keywords must remain exported to:
- Maintain public API surface for downstream consumers
- Preserve correct scoping in API documentation
- Keep original type semantics intact

Author: privatenumber

src/transform/preprocess.ts

@@ -23,10 +23,6 @@ function preProcessNamespaceBody(body: ts.ModuleBlock, code: MagicString, source
     // Safely call the new context-aware function on all children
     fixModifiers(code, stmt);
 
-    if (ts.isModuleDeclaration(stmt)) {
-      duplicateExports(code, stmt);
-    }
-
     // Recurse for nested namespaces
     if (ts.isModuleDeclaration(stmt) && stmt.body && ts.isModuleBlock(stmt.body)) {
       preProcessNamespaceBody(stmt.body, code, sourceFile);
@@ -593,16 +589,8 @@ function fixModifiers(code: MagicString, node: ts.Node) {
     if (needsDeclare && !hasDeclare) {
       code.appendRight(node.getStart(), "declare ");
     }
-  } else {
-    // For statements inside a namespace/module, only remove `export` from a nested namespace
-    // Leave all other members (vars, types, etc.) untouched
-    if (ts.isModuleDeclaration(node)) {
-      const exportKeyword = node.modifiers?.find((mod) => mod.kind === ts.SyntaxKind.ExportKeyword);
-      if (exportKeyword) {
-        code.remove(exportKeyword.getStart(), exportKeyword.getEnd() + 1);
-      }
-    }
   }
+  // For statements inside namespaces, preserve all modifiers (including export)
 }
 
 function duplicateExports(code: MagicString, module: ts.ModuleDeclaration) {

tests/testcases/namespace-nested-export/expected.d.ts

@@ -2,7 +2,7 @@ interface External {
   id: string;
 }
 declare namespace Container {
-  namespace Nested {
+  export namespace Nested {
     // Import inside nested namespace with export keyword preserved
     import Local = External;
     export type FinalType = Local;

tests/testcases/namespace-nested-shadowing/expected.d.ts

@@ -2,7 +2,7 @@ interface TargetType {
   version: 'v2';
 }
 declare namespace Outer {
-  namespace Inner {
+  export namespace Inner {
     // Import shadows outer interface, verifies scoping and tree-shaking
     import ShadowedType = TargetType;
     export type Result = ShadowedType;

tests/testcases/namespace-reexport-interface/expected.d.ts

@@ -4,7 +4,7 @@ declare namespace ArrayObjectStore {
     id: string;
     value: number;
   }
-  namespace Util {
+  export namespace Util {
     export function helper(): void;
   }
 }