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

Buffer.h

@@ -334,6 +334,7 @@ public:
 	{
 		filename_       = name;
 		is_file_backed_ = false;
+		is_virtual_     = true;
 	}
 
 
@@ -343,6 +344,14 @@ public:
 	}
 
 
+	// A virtual buffer (e.g. "+HELP+") has a display name that is not a path:
+	// it is never journaled, and saving it asks for a file name.
+	[[nodiscard]] bool IsVirtual() const
+	{
+		return is_virtual_;
+	}
+
+
 	[[nodiscard]] bool Dirty() const
 	{
 		return dirty_;
@@ -677,6 +686,7 @@ private:
 
 	std::string filename_;
 	bool is_file_backed_              = false;
+	bool is_virtual_                  = false;
 	bool dirty_                       = false;
 	bool read_only_                   = false;
 	bool mark_set_                    = false;