fix(javascript): resolve package exports targets only to the exact file (#1181)

A selected package exports target names one file, and Node tries no
extension or directory index for it. REA still resolved it like a relative
specifier, so `exports: "./file"` with only `file.js` present, or
`"./folder"` with only `folder/index.js`, reported a resolved import that
Node rejects under both import and require.

Resolve an exports target only to its exact file and otherwise report
not-found with a limitation naming the target. Legacy main entries keep
their extension and index fallbacks.

Fixes #1174

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/application/javascript/JavaScriptArtifactPathResolution.ts

@@ -105,8 +105,11 @@ const resolvePackagePath = (
         ],
       },
     );
-  const resolved =
-    mode === "files-and-index"
+  // An exports target names one exact file; Node tries no extension or index.
+  const resolved = exportsTargetUrl
+    ? (resolveFileCandidates(input, [confined]) ??
+      exactExportsTargetNotFound(confined))
+    : mode === "files-and-index"
       ? (resolveFileCandidates(input, [
           ...fileCandidates(confined),
           ...indexCandidates(confined),
@@ -619,6 +622,14 @@ const notFoundCandidate = (): CandidateResolution => ({
   ],
 });
 
+const exactExportsTargetNotFound = (path: string): CandidateResolution => ({
+  resolvedPath: null,
+  status: "not-found",
+  limitations: [
+    `The selected package exports target ${path} does not exist in the inventoried artifact container; exports targets are exact files, so no extension or directory index was tried.`,
+  ],
+});
+
 const fileUrlPath = (value: string): string | undefined => {
   if (!value.startsWith("file://")) return undefined;
   try {