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_detect.py

@@ -198,6 +198,30 @@ class TestGuessContent:
         assert detect_type(content=sample_json, guess_content=True) == "json"
 
 
+class TestDebugLogging:
+    """Debug logging describes inference decisions."""
+
+    def test_logs_extension_source(self) -> None:
+        messages: list[str] = []
+
+        assert detect_type(path="script.py", debug=messages.append) == "python"
+        assert messages == ["type: inferred 'python' from extension '.py'"]
+
+    def test_logs_content_guess_and_score(self) -> None:
+        messages: list[str] = []
+        diff = "--- a/file\n+++ b/file\n@@ -1 +1 @@\n-old\n+new\n"
+
+        assert (
+            detect_type(content=diff, guess_content=True, debug=messages.append)
+            == "diff"
+        )
+        assert any(
+            "type: content guess -> lexer='diff', score=" in message
+            for message in messages
+        )
+        assert "type: inferred 'diff' from heuristic content guessing" in messages
+
+
 class TestFallback:
     """Priority 7: fallback to 'text'."""