fix(router-plugin): respect directive prologues when inserting imports

Add insertAfterDirectives() helper to ensure imports are inserted after
directive prologues ('use client', 'use server', etc.) rather than before
them. This is critical for React Server Components and other frameworks
that require directives to be the first statements in a module.

Previously, unshiftContainer() would insert imports at the start of the
program body, potentially before directives, which would cause them to be
ignored or throw errors.

All 5 usages of unshiftContainer() have been replaced with the new helper:
- LazyRouteComponent imports
- Dynamic import importer functions
- lazyFn imports
- Shared variable imports (with correct index tracking)

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

Co-Authored-By: Claude <[email protected]>

Author: KyleAMathews

packages/router-plugin/src/core/code-splitter/compilers.ts

@@ -146,6 +146,34 @@ function isTopLevelVarDecl(
   return false
 }
 
+/**
+ * Insert nodes into the program body after any directive prologues ('use client', 'use server', etc.)
+ * This ensures directives remain first in the file, which is required by frameworks like React Server Components.
+ * Returns the index where the nodes were inserted.
+ */
+function insertAfterDirectives(
+  programPath: babel.NodePath<t.Program>,
+  nodes: Array<t.Statement> | t.Statement,
+): number {
+  const body = programPath.get('body') as Array<babel.NodePath>
+  let insertIndex = 0
+
+  // Find the first non-directive statement
+  while (
+    insertIndex < body.length &&
+    body[insertIndex]!.isExpressionStatement() &&
+    t.isDirectiveLiteral((body[insertIndex]!.node as any).expression)
+  ) {
+    insertIndex++
+  }
+
+  // Insert at the calculated position
+  const nodesToInsert = Array.isArray(nodes) ? nodes : [nodes]
+  programPath.node.body.splice(insertIndex, 0, ...nodesToInsert)
+
+  return insertIndex
+}
+
 export function compileCodeSplitReferenceRoute(
   opts: ParseAstOptions & {
     codeSplitGroupings: CodeSplitGroupings
@@ -411,7 +439,7 @@ export function compileCodeSplitReferenceRoute(
                             LAZY_ROUTE_COMPONENT_IDENT,
                           )
                         ) {
-                          programPath.unshiftContainer('body', [
+                          insertAfterDirectives(programPath, [
                             template.statement(
                               `import { ${LAZY_ROUTE_COMPONENT_IDENT} } from '${PACKAGE}'`,
                             )(),
@@ -425,7 +453,7 @@ export function compileCodeSplitReferenceRoute(
                             splitNodeMeta.localImporterIdent,
                           )
                         ) {
-                          programPath.unshiftContainer('body', [
+                          insertAfterDirectives(programPath, [
                             template.statement(
                               `const ${splitNodeMeta.localImporterIdent} = () => import('${splitUrl}')`,
                             )(),
@@ -479,8 +507,8 @@ export function compileCodeSplitReferenceRoute(
 
                         // Prepend the import statement to the program along with the importer function
                         if (!hasImportedOrDefinedIdentifier(LAZY_FN_IDENT)) {
-                          programPath.unshiftContainer(
-                            'body',
+                          insertAfterDirectives(
+                            programPath,
                             template.smart(
                               `import { ${LAZY_FN_IDENT} } from '${PACKAGE}'`,
                             )(),
@@ -494,7 +522,7 @@ export function compileCodeSplitReferenceRoute(
                             splitNodeMeta.localImporterIdent,
                           )
                         ) {
-                          programPath.unshiftContainer('body', [
+                          insertAfterDirectives(programPath, [
                             template.statement(
                               `const ${splitNodeMeta.localImporterIdent} = () => import('${splitUrl}')`,
                             )(),
@@ -1100,10 +1128,12 @@ export function compileCodeSplitVirtualRoute(
                 removeSplitSearchParamFromFilename(opts.filename),
               ),
             )
-            programPath.unshiftContainer('body', importDecl)
+            const importIndex = insertAfterDirectives(programPath, importDecl)
 
             // Track imported identifiers for dead code elimination
-            const importPath = programPath.get('body')[0] as babel.NodePath
+            const importPath = programPath.get('body')[
+              importIndex
+            ] as babel.NodePath
             importPath.traverse({
               Identifier(identPath) {
                 if (