Terminal loop, virtual buffers, yank bound

- kte refuses to start when stdin is not a terminal: piped bytes were
  executed as editor commands (C-k s plus a path writes a file), and
  after EOF the loop spun at full CPU with no way to quit.
- The terminal frontend handles all pending input before drawing (a
  paste used to cost a frame and a full redraw per character) and
  redraws only after input, a resize or deferred opens, else once a
  second. The renderer caches, per row and buffer version, where the
  horizontally scrolled part of a line starts and the cursor's render
  column, instead of rescanning the line every frame. Idle CPU with the
  cursor at the end of a 20 MB line: one full core -> ~1%.
- Buffers with a virtual name (+HELP+) are not journaled and saving one
  asks for a file name; it used to write ./+HELP+ and journal under
  that path.
- Yank with a repeat count is capped at 256 MiB in total instead of
  reserving count x size bytes (C-u 1000000 C-y of a large kill).
- Remove an overload clang reports as unused (-Werror on macOS).

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

tests/test_audit_regressions.cc

@@ -9,6 +9,7 @@
 #include "ErrorRecovery.h"
 #include "PieceTable.h"
 #include "RegexGuard.h"
+#include "Swap.h"
 #include "UndoTree.h"
 #include "syntax/HighlighterEngine.h"
 #include "syntax/HighlighterRegistry.h"
@@ -992,3 +993,36 @@ TEST(Audit_Save_KeepsSpecialModeBits)
 	ASSERT_EQ(::stat(f.c_str(), &st), 0);
 	ASSERT_EQ(st.st_mode & 07777, (mode_t) 06755);
 }
+
+
+// The help buffer's name is not a path: it is not journaled, and saving it
+// asks for a file name instead of writing ./+HELP+.
+TEST(Audit_HelpBuffer_NotJournaledOrSavedByName)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	ASSERT_TRUE(h.Exec(CommandId::ShowHelp));
+	Buffer *help = ed.CurrentBuffer();
+	ASSERT_TRUE(help->IsVirtual());
+	ASSERT_TRUE(h.Exec(CommandId::ToggleReadOnly));
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "x"));
+	ed.Swap()->Flush(help);
+	ASSERT_TRUE(!std::filesystem::exists(kte::SwapManager::ComputeSwapPathForTests(*help)));
+	ASSERT_TRUE(h.Exec(CommandId::Save));
+	ASSERT_TRUE(ed.PromptActive());
+	ASSERT_TRUE(ed.CurrentPromptKind() == Editor::PromptKind::SaveAs);
+	ASSERT_TRUE(!std::filesystem::exists("+HELP+"));
+	ASSERT_TRUE(h.Exec(CommandId::Refresh));
+}
+
+
+// A huge repeated yank is refused rather than allocating without bound.
+TEST(Audit_Yank_HugeRepeatRefused)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	ed.KillRingPush(std::string(1 << 20, 'k'));
+	ASSERT_TRUE(!h.Exec(CommandId::Yank, std::string(), 1000000));
+	ASSERT_EQ(h.Buf().Nrows(), (std::size_t) 1);
+	ASSERT_EQ(h.Line(0), std::string());
+}