Fall back to focused tab when getReaperParent(3) is null

REAPER may not expose the parent project pointer during plugin creation/setState, so the instrument would silently resolve no sample roots. Only that null-parent case now falls back to EnumProjects(-1); a resolved-but-unlisted or unsaved project still resolves nothing.

src/core/instrument/map/sample_roots.cpp

@@ -6,10 +6,12 @@
 
 namespace reasampler::instrument::map {
 
-std::string ownProjectDir(const void* own, const std::vector<ProjectTab>& open) {
-    if (!own) return {};
+std::string ownProjectDir(const void* own, const std::vector<ProjectTab>& open,
+                          const void* activeFallback) {
+    const void* target = own ? own : activeFallback;
+    if (!target) return {};
     for (const ProjectTab& tab : open) {
-        if (tab.project == own) return capture::projectDirOfRpp(tab.rppPath);
+        if (tab.project == target) return capture::projectDirOfRpp(tab.rppPath);
     }
     return {};
 }