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.

tests/test_cli.py

@@ -75,6 +75,7 @@ class TestFileInput:
             content="x = 1\n",
             explicit_type=None,
             guess_content=False,
+            debug=None,
         )
 
     def test_render_called_with_correct_options(self, tmp_file) -> None:
@@ -182,8 +183,41 @@ class TestStdinInput:
             content="hello\n",
             explicit_type=None,
             guess_content=False,
+            debug=None,
         )
 
+    def test_debug_env_enables_debug_logger(self) -> None:
+        with patch("rp.cli.detect_type") as mock_detect_type:
+            mock_detect_type.return_value = "text"
+            with patch("rp.cli.render"):
+                runner.invoke(
+                    app, ["--no-pager"], input="hello\n", env={"RP_DEBUG": "1"}
+                )
+
+        _, kwargs = mock_detect_type.call_args
+        assert kwargs["debug"] is not None
+
+    def test_debug_output_stays_on_stderr(self, tmp_file) -> None:
+        file = tmp_file("# Hello\n", suffix=".md")
+
+        result = runner.invoke(app, [str(file), "--no-pager"], env={"RP_DEBUG": "1"})
+        normalized_stderr = result.stderr.replace("\n", "")
+
+        assert result.exit_code == 0
+        assert "[rp debug]" not in result.stdout
+        assert "Hello" in result.stdout
+        assert "[rp debug] input: reading file" in result.stderr
+        assert repr(str(file)) in normalized_stderr
+        assert (
+            "[rp debug] type: inferred 'markdown' from extension '.md'" in result.stderr
+        )
+        assert (
+            "[rp debug] render: auto-set line_numbers=False for 'markdown'"
+            in result.stderr
+        )
+        assert "[rp debug] pager: explicitly disabled" in result.stderr
+        assert "[rp debug] pager: writing directly to the console" in result.stderr
+
     def test_stdin_shows_tty_prompt(self) -> None:
         """When stdin is a TTY and no input is piped, show the reading hint."""
         with patch("rp.cli.sys.stdin.isatty", return_value=True):