Fix multi-window architecture and swap file cleanup

Multi-window:
- Per-window ImGui contexts (fixes input, scroll, and rendering isolation)
- Per-instance scroll and mouse state in ImGuiRenderer (no more statics)
- Proper GL context activation during window destruction
- ValidateBufferIndex guards against stale curbuf_ across shared buffers
- Editor methods (CurrentBuffer, SwitchTo, CloseBuffer, etc.) use Buffers()
  accessor to respect shared buffer lists
- New windows open with an untitled buffer
- Scratch buffer reuse works in secondary windows
- CMD-w on macOS closes only the focused window
- Deferred new-window creation to avoid mid-frame ImGui context corruption

Swap file cleanup:
- SaveAs prompt handler now calls ResetJournal
- cmd_save_and_quit now calls ResetJournal
- Editor::Reset detaches all buffers before clearing
- Tests for save-and-quit and editor-reset swap cleanup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Editor.h

@@ -521,7 +521,7 @@ public:
 	// Buffers
 	[[nodiscard]] std::size_t BufferCount() const
 	{
-		return buffers_.size();
+		return Buffers().size();
 	}
 
 
@@ -531,6 +531,19 @@ public:
 	}
 
 
+	// Clamp curbuf_ to valid range. Call when the shared buffer list may
+	// have been modified by another editor (e.g., buffer closed in another window).
+	void ValidateBufferIndex()
+	{
+		const auto &bufs = Buffers();
+		if (bufs.empty()) {
+			curbuf_ = 0;
+		} else if (curbuf_ >= bufs.size()) {
+			curbuf_ = bufs.size() - 1;
+		}
+	}
+
+
 	Buffer *CurrentBuffer();
 
 	const Buffer *CurrentBuffer() const;