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.

tests/test_sample_roots.cpp

@@ -64,6 +64,18 @@ static void testNoOpenProjectsResolvesNothing() {
     CHECK(ownProjectDir(&projA, {}).empty());
 }
 
+static void testNullOwnFallsBackToTheActiveTab() {
+    CHECK(ownProjectDir(nullptr, threeTabs(), &projB) == "c:/projects/b");
+}
+
+static void testNullOwnAndNullFallbackResolvesNothing() {
+    CHECK(ownProjectDir(nullptr, threeTabs(), nullptr).empty());
+}
+
+static void testNonNullOwnIgnoresTheFallback() {
+    CHECK(ownProjectDir(&stranger, threeTabs(), &projB).empty());
+}
+
 // --- sampleRootCandidates -----------------------------------------------------
 
 static void testProjectDirIsTheOnlyCandidate() {
@@ -141,6 +153,9 @@ int main() {
     testUnlistedProjectResolvesNothingRatherThanAnotherTab();
     testUnsavedOwnProjectResolvesNothingEvenBesideSavedTabs();
     testNoOpenProjectsResolvesNothing();
+    testNullOwnFallsBackToTheActiveTab();
+    testNullOwnAndNullFallbackResolvesNothing();
+    testNonNullOwnIgnoresTheFallback();
 
     testProjectDirIsTheOnlyCandidate();
     testNoProjectDirMeansNoCandidates();