Add opt-in debug logging across the CLI pipeline

Thread RP_DEBUG-driven logging through input, detection, rendering, and pager decisions so CLI behavior is inspectable without polluting stdout. Add focused coverage for debug wiring, env parsing, and stderr-only output.

src/rp/cli.py

@@ -10,6 +10,7 @@ import typer
 from rich.console import Console
 
 from rp import __author__, __license__, __version__
+from rp.debug import env_debug_enabled, log_debug, make_debug_logger
 from rp.detect import detect_type
 from rp.render import RenderOptions, render
 
@@ -95,13 +96,16 @@ def main(
         version: Show version and exit.
     """
     console = Console()
+    debug = make_debug_logger(Console(stderr=True), env_debug_enabled())
 
     if file is None or str(file) == "-":
+        log_debug(debug, "input: reading from stdin")
         if sys.stdin.isatty():
             console.print("[dim]Reading from stdin. Press Ctrl+D to end.[/dim]")
         content = sys.stdin.read()
         file_path = None
     else:
+        log_debug(debug, f"input: reading file {str(file)!r}")
         if not file.exists():
             console.print(f"[red]Error:[/red] File not found: {file}")
             raise typer.Exit(code=1)
@@ -127,12 +131,14 @@ def main(
         content=content,
         explicit_type=file_type,
         guess_content=guess_content,
+        debug=debug,
     )
 
     options = RenderOptions(
         theme=theme,
         line_numbers=line_numbers,
         pager=pager,
+        debug=debug,
     )
     render(content, file_type, console, options)