Account for every uncovered line in the implementation

Closes #23

ConflictSet.cpp

@@ -2395,7 +2395,11 @@ Iterator firstGeq(Node *n, const std::span<const uint8_t> key) {
       } else {
         n = nextSibling(n);
         if (n == nullptr) {
-          return {nullptr, 1};
+          //  This line is genuinely unreachable from any entry point of the
+          //  final library, since we can't remove a key without introducing a
+          //  key after it, and the only production caller of firstGeq is for
+          //  resuming the setOldestVersion scan.
+          return {nullptr, 1}; // GCOVR_EXCL_LINE
         }
         goto downLeftSpine;
       }

test_conflict_set.py

@@ -92,6 +92,14 @@ def test_large():
         )
 
 
+def test_merge_child_node48():
+    with DebugConflictSet() as cs:
+        cs.addWrites(1, write(b"\x00" * 9))
+        for i in range(17):
+            cs.addWrites(1, write(b"\x00" * 10 + bytes([i])))
+        cs.addWrites(1, write(b"\x00" * 8, b"\x00" * 10))
+
+
 if __name__ == "__main__":
     # budget "pytest" for ctest integration without pulling in a dependency. You can of course still use pytest in local development.
     import argparse