Resolve ReaSampler 9000 samples against the instance's own project, not the focused tab

sampleRoots matches getReaperParent(3) among EnumProjects' open projects; every instrument sample read goes through an ordered root list (one root today) so a later temp-bank root slots in after it.

src/core/instrument/map/sample_roots.h

@@ -0,0 +1,41 @@
+// sample_roots — where the instrument looks for a bank-relative sample: the directory of
+// the project it LIVES IN, found by identity among the open projects, never the focused
+// tab's. Pure: the shell enumerates REAPER's projects and reads files; this decides which
+// project, which roots, and in what order.
+
+#pragma once
+
+#include <cstdint>
+#include <functional>
+#include <string>
+#include <vector>
+
+namespace reasampler::instrument::map {
+
+// One enumerated project: its identity (an opaque ReaProject*, compared, never
+// dereferenced) and its .rpp path, empty for a never-saved project.
+struct ProjectTab {
+    const void* project = nullptr;
+    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 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
+// there is no project directory, which every caller treats as nothing to play.
+std::vector<std::string> sampleRootCandidates(const std::string& projectDir);
+
+using FileReader = std::function<std::vector<std::uint8_t>(const std::string& path)>;
+
+// The bytes of `relativePath` resolved under the first root whose read yields any (a read
+// that comes back empty falls through to the next root); empty when none does.
+std::vector<std::uint8_t> readFromFirstRoot(const std::vector<std::string>& roots,
+                                            const std::string& relativePath,
+                                            const FileReader& read);
+
+}  // namespace reasampler::instrument::map