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.h

@@ -19,11 +19,17 @@ struct ProjectTab {
     std::string rppPath;
 };
 
-// The directory of the project whose identity is `own`; empty when `own` is null, is not
-// among `open`, or was never saved. Deliberately no fallback to any other project: another
-// project's bank folder can hold a same-named file, and playing it is the wrong-project
-// bug this exists to close.
-std::string ownProjectDir(const void* own, const std::vector<ProjectTab>& open);
+// The directory of the project whose identity is `own`; empty when `own` is not among
+// `open` or was never saved. Deliberately no fallback to any other project for a
+// NON-NULL `own`: another project's bank folder can hold a same-named file, and playing
+// it is the wrong-project bug this exists to close.
+//
+// `activeFallback` is used ONLY when `own` is null (REAPER may not expose
+// getReaperParent(3) yet during plugin creation/setState) — then it stands in for `own`
+// unchanged, so a still-null fallback (or one absent from `open`) resolves nothing exactly
+// as a null `own` always did.
+std::string ownProjectDir(const void* own, const std::vector<ProjectTab>& open,
+                          const void* activeFallback = nullptr);
 
 // The roots a bank-relative path is tried against, in order. Only the project directory
 // today — the ordered list is the seam a later root (tried after it) extends. Empty when