Fix critical PPX bug: DOM element externals not injected at structure level

**Root Cause**: External declarations for DOM elements with data attributes
were generated at expression level but only injected at structure level,
causing "Unbound value makeProps_div_*" compilation errors.

**Technical Analysis**:
- DOM elements processed in `transformLowercaseCall3` (expression level)
- External declarations stored in `externalDeclarations` ref
- But externals only injected in `method! structure` which resets the ref
- Result: DOM element externals never reach final AST output

**Solution**: Modified structure processing to preserve externals across
nested contexts by:
1. Storing parent externals before processing
2. Collecting all externals (React components + DOM elements)
3. Restoring and accumulating externals properly
4. Injecting all externals at structure boundaries

**Maintains**:
- Existing functionality for React components
- Deduplication logic (externalExists check)
- Zero-runtime performance (compile-time only)
- Kebab-case transformation (data_test_id → data-test-id)

**Fixes**: All DOM elements with data attributes now compile successfully
including nested modules, functions, and complex nested structures.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: johnhaley81

demo/main.re

@@ -221,6 +221,7 @@ module WithoutForward = {
   };
 };
 
+
 module DataAttrsDemo = {
   [@react.component]
   let make = () => {
@@ -233,7 +234,7 @@ module DataAttrsDemo = {
       </div>
 
       <div 
-        className="demo-multi"
+        className="demo-multi" data_testid="demo-multi" data_value="test"
       >
         {React.string("Data attributes: data_testid becomes data-testid")}
       </div>
@@ -267,6 +268,7 @@ module DataAttrsDemo = {
   };
 };
 
+
 module App = {
   [@react.component]
   let make = (~initialValue) => {

ppx/reason_react_ppx.ml

@@ -1464,14 +1464,21 @@ let jsxMapper =
     [@@raises Invalid_argument]
 
     method! structure ctxt stru =
+      (* Store existing externals from any parent context *)
+      let parentExternals = !externalDeclarations in
       externalDeclarations := [];
 
+      (* Process the structure, collecting React component externals *)
       let processedStru = super#structure ctxt (reactComponentTransform ~ctxt self stru) in
 
-      let externals = List.rev !externalDeclarations in
-      externalDeclarations := [];
+      (* Collect externals from both React components and DOM elements *)
+      let allExternals = List.rev !externalDeclarations in
+      
+      (* Restore parent externals and add our collected externals *)
+      externalDeclarations := allExternals @ parentExternals;
 
-      externals @ processedStru
+      (* Inject externals at the beginning of this structure *)
+      allExternals @ processedStru
     [@@raises Invalid_argument]
 
     method! expression ctxt expr =