fix(reference): preserve missing symlink target facts

Treat ENOTDIR while resolving a symlink target as missing target evidence, preserving its spelling and partial inventory. Keep graph-level excluded-ancestor rejection separate from import success. The compiled boundary regression passes.

src/reference/ReferenceSourceReaderEntries.ts

@@ -357,7 +357,7 @@ const readSymlinkTarget = async (
   } catch (cause: unknown) {
     const code =
       cause instanceof Error ? Reflect.get(cause, "code") : undefined;
-    if (code !== "ENOENT") throw cause;
+    if (code !== "ENOENT" && code !== "ENOTDIR") throw cause;
     return {
       status: "read",
       kind: "symlink",