Route every extension bank-directory lookup through one bankRootOf seam

The pure bankRootFor(rppPath, unsavedRoot) and the shell bankRootOf(ReaProject*) replace eleven hand-rolled .rpp-parent derivations. No unsaved root is supplied yet, so behaviour and refusals are unchanged. Save-As relocation now reads the stored lastBankRoot_.

tests/test_capture_paths.cpp

@@ -402,6 +402,49 @@ static void testBankRelativeForNameConventionAndEdge() {
     CHECK(bankRelativeForName("").empty());
 }
 
+// --- bankRootFor --------------------------------------------------------------
+
+static void testBankRootForSavedProjectIsTheRppDirectory() {
+    CHECK(bankRootFor("/home/d/proj/song.rpp", "") == "/home/d/proj");
+#ifdef _WIN32
+    CHECK(bankRootFor("C:\\Users\\D\\Proj\\song.rpp", "") == "c:/users/d/proj");
+#endif
+}
+
+static void testBankRootForSavedProjectIgnoresTheUnsavedRoot() {
+    // Once the project has a directory, a supplied unsaved root must not leak through.
+    CHECK(bankRootFor("/home/d/proj/song.rpp", "/tmp/unsaved") == "/home/d/proj");
+}
+
+static void testBankRootForUnsavedProjectFallsBackToTheUnsavedRoot() {
+    CHECK(bankRootFor("", "/tmp/unsaved") == "/tmp/unsaved");
+    // Normalized like the saved branch: trailing slash stripped, backslashes flipped.
+    CHECK(bankRootFor("", "/tmp/unsaved/") == "/tmp/unsaved");
+#ifdef _WIN32
+    CHECK(bankRootFor("", "C:\\Temp\\Unsaved\\") == "c:/temp/unsaved");
+#endif
+}
+
+static void testBankRootForUnsavedProjectWithNoUnsavedRootIsEmpty() {
+    // No CWD / default-location fallback: an unsaved project with nowhere to bank
+    // yields empty, which every caller treats as "no bank location".
+    CHECK(bankRootFor("", "").empty());
+}
+
+static void testBankRootForRppWithNoDirectoryIsTreatedAsUnsaved() {
+    CHECK(bankRootFor("song.rpp", "/tmp/unsaved") == "/tmp/unsaved");
+    CHECK(bankRootFor("song.rpp", "").empty());
+}
+
+static void testBankRootForBothBranchesProduceComparableRoots() {
+    // The same directory reached through either branch is the same string, so a
+    // root recorded under one branch compares equal to the other's.
+    CHECK(bankRootFor("/a/b/song.rpp", "") == bankRootFor("", "/a/b/"));
+#ifdef _WIN32
+    CHECK(bankRootFor("C:\\A\\B\\song.rpp", "") == bankRootFor("", "c:/a/b"));
+#endif
+}
+
 int main() {
     testNormalizeSlashes();
     testNormalizeSlashesCaseFolding();
@@ -433,6 +476,12 @@ int main() {
     testRenderPathsTakeTheirDirectoryVerbatim();
     testRenderPathsEmptyDirectoryStaysEmpty();
     testBankRelativeForNameConventionAndEdge();
+    testBankRootForSavedProjectIsTheRppDirectory();
+    testBankRootForSavedProjectIgnoresTheUnsavedRoot();
+    testBankRootForUnsavedProjectFallsBackToTheUnsavedRoot();
+    testBankRootForUnsavedProjectWithNoUnsavedRootIsEmpty();
+    testBankRootForRppWithNoDirectoryIsTreatedAsUnsaved();
+    testBankRootForBothBranchesProduceComparableRoots();
 
     if (g_fail == 0) std::printf("capture_paths: all tests passed\n");
     else std::printf("capture_paths: %d CHECK(s) FAILED\n", g_fail);