perf: skip rewriting files untouched by in-place anonymization (#22)

In-place anonymization always replaced the destination with the temp
file even when no line changed, rewriting every clean trace and
churning mtimes. Track whether any line was actually modified and drop
the temp file when an in-place run left the content identical; apply
the same check to whole-text (non-jsonl) files.

Co-authored-by: CTO Agent <cto@paperclip.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

src/teich/anonymize.py

@@ -107,7 +107,8 @@ def _anonymize_file(source: Path, destination: Path) -> AnonymizeFileReport:
                 shutil.copy2(source, destination)
             return AnonymizeFileReport(path=source, output_path=destination)
         text = anonymizer.anonymize_text(original)
-        destination.write_text(text, encoding="utf-8")
+        if text != original or source.resolve() != destination.resolve():
+            destination.write_text(text, encoding="utf-8")
 
     return AnonymizeFileReport(
         path=source,
@@ -132,6 +133,7 @@ def _anonymize_jsonl_file(source: Path, destination: Path, anonymizer: "TraceAno
             ) as temp_handle,
         ):
             temp_path = Path(temp_handle.name)
+            changed = False
             for raw_line in handle:
                 stripped = raw_line.strip()
                 if not stripped:
@@ -140,12 +142,16 @@ def _anonymize_jsonl_file(source: Path, destination: Path, anonymizer: "TraceAno
                 try:
                     value = json.loads(raw_line)
                 except json.JSONDecodeError:
-                    temp_handle.write(anonymizer.anonymize_text(raw_line))
+                    anonymized_line = anonymizer.anonymize_text(raw_line)
+                    if anonymized_line != raw_line:
+                        changed = True
+                    temp_handle.write(anonymized_line)
                     continue
                 anonymized = anonymizer.anonymize_value(value)
                 if anonymized == value:
                     temp_handle.write(raw_line)
                     continue
+                changed = True
                 line = json.dumps(anonymized, ensure_ascii=False, separators=(",", ":"))
                 line = line.replace("\u0085", "\\u0085").replace("\u2028", "\\u2028").replace("\u2029", "\\u2029")
                 temp_handle.write(line + "\n")
@@ -161,7 +167,10 @@ def _anonymize_jsonl_file(source: Path, destination: Path, anonymizer: "TraceAno
         )
         return
     if temp_path is not None:
-        temp_path.replace(destination)
+        if not changed and source.resolve() == destination.resolve():
+            temp_path.unlink()
+        else:
+            temp_path.replace(destination)
 
 
 class TraceAnonymizer: