Editor: share the SwapManager along with shared buffers

A secondary GUI window shared the primary's buffer vector but attached
buffers through its own SwapManager. Closing the window destroyed that
manager while buffers still held its recorders, so the next edit called
into freed memory; Rehome through the wrong manager also dropped
journaling for the primary's buffers.

Audit finding M3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

tests/test_audit_regressions.cc

@@ -11,6 +11,7 @@
 #include "UndoTree.h"
 
 #include <chrono>
+#include <memory>
 #include <string>
 #include <thread>
 
@@ -225,3 +226,25 @@ TEST(Audit_D8_CircuitBreaker_WindowExpires)
 	ASSERT_TRUE(cb.GetState() == kte::CircuitBreaker::State::Closed);
 	ASSERT_EQ(cb.GetFailureCount(), (std::size_t) 1);
 }
+
+
+// M3: an editor that shares another's buffers must attach them through the
+// owner's SwapManager, so destroying it leaves no dangling recorders.
+TEST(Audit_M3_SharedBuffers_UseOwnersSwapManager)
+{
+	TestHarness h;
+	Editor &primary = h.EditorRef();
+
+	auto secondary = std::make_unique<Editor>();
+	secondary->SetSharedBuffers(&primary.Buffers(), primary.Swap());
+	ASSERT_TRUE(secondary->Swap() == primary.Swap());
+	secondary->AddBuffer(Buffer());
+	const std::size_t idx = primary.Buffers().size() - 1;
+	secondary.reset();
+
+	// Edits record through the buffer's swap recorder; before the fix it
+	// pointed into the destroyed secondary SwapManager.
+	Buffer &b = primary.Buffers()[idx];
+	b.insert_text(0, 0, "abc");
+	ASSERT_EQ(b.GetLineString(0), std::string("abc"));
+}