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>

Command.cc

@@ -752,6 +752,8 @@ cmd_save_and_quit(CommandContext &ctx)
 		if (buf->IsFileBacked()) {
 			if (buf->Save(err)) {
 				buf->SetDirty(false);
+				if (auto *sm = ctx.editor.Swap())
+					sm->ResetJournal(*buf);
 			} else {
 				ctx.editor.SetStatus(err);
 				return false;
@@ -759,6 +761,8 @@ cmd_save_and_quit(CommandContext &ctx)
 		} else if (!buf->Filename().empty()) {
 			if (buf->SaveAs(buf->Filename(), err)) {
 				buf->SetDirty(false);
+				if (auto *sm = ctx.editor.Swap())
+					sm->ResetJournal(*buf);
 			} else {
 				ctx.editor.SetStatus(err);
 				return false;
@@ -2568,6 +2572,10 @@ cmd_newline(CommandContext &ctx)
 							ctx.editor.SetStatus(err);
 						} else {
 							buf->SetDirty(false);
+							if (auto *sm = ctx.editor.Swap()) {
+								sm->NotifyFilenameChanged(*buf);
+								sm->ResetJournal(*buf);
+							}
 							ctx.editor.SetStatus("Saved as " + value);
 							if (auto *u = buf->Undo())
 								u->mark_saved();