fix: report renames honestly in a dry run

--dry-run described a FAT32 rename as an encode. process() skipped the rename
whenever dry_run was set, then found no file at the target and fell through to
the encode path, so a preview of enabling --fat32-safe announced a full
re-encode of every track whose name held a reserved character. The real run
moves those files in a moment. A preview that inverts the cost of the thing
being previewed is worse than no preview at all.

Prune compounded it. With nothing renamed, the pre-sanitisation files are still
on disk, and they were reported as orphans due for deletion -- so the same dry
run claimed the library would be re-encoded and the originals thrown away,
neither of which is true.

Renames are now their own outcome: reported as "would rename" in a dry run,
counted separately from encodes in the pass summary, and excluded from the
orphan list when a dry run leaves them in place.

music_mirror.py

@@ -357,7 +357,7 @@ def copy(source, mirror, dry_run):
     return Result("copied", mirror)
 
 
-def adopt_existing(source, mirror, previous):
+def adopt_existing(source, mirror, previous, dry_run=False):
     """Move an already-encoded file to its new name. Returns whether it moved.
 
     Turning on FAT32-safe naming changes the path of every track whose name
@@ -367,6 +367,9 @@ def adopt_existing(source, mirror, previous):
     """
     if previous == mirror or not previous.is_file() or not is_current(source, previous):
         return False
+    if dry_run:
+        logger.info("would rename %s -> %s", previous.name, mirror.name)
+        return True
     mirror.parent.mkdir(parents=True, exist_ok=True)
     os.replace(previous, mirror)
     logger.info("renamed %s -> %s", previous.name, mirror.name)
@@ -375,8 +378,12 @@ def adopt_existing(source, mirror, previous):
 
 def process(source, mirror, quality_args, dry_run, previous=None):
     """Bring one source file's mirror entry up to date."""
-    if previous is not None and not dry_run and not mirror.exists():
-        adopt_existing(source, mirror, previous)
+    # Counted separately from an encode, and reported in a dry run, because the
+    # difference between moving a file and re-encoding it is the difference
+    # between a minute and an afternoon.
+    if previous is not None and not mirror.exists():
+        if adopt_existing(source, mirror, previous, dry_run):
+            return Result("renamed", mirror)
     if is_current(source, mirror):
         # A mirror written before this bit was set has a correct mtime, so
         # nothing else in the pass would ever revisit it. Top it up here
@@ -483,7 +490,7 @@ def run_once(
     """
     started = time.monotonic()
     logger.info("pass starting with %d concurrent encoders", jobs)
-    counts = {"encoded": 0, "copied": 0, "skipped": 0, "failed": 0}
+    counts = {"encoded": 0, "copied": 0, "renamed": 0, "skipped": 0, "failed": 0}
     failures = []
 
     work = plan(scan_root, source_root, mirror_root, safe)
@@ -506,16 +513,27 @@ def run_once(
             if result.action == "failed":
                 failures.append(result)
 
-    removed = prune(mirror_root, set(work), dry_run) if do_prune else 0
+    expected = set(work)
+    if safe and dry_run:
+        # Nothing was actually renamed, so the pre-sanitisation files are still
+        # on disk. They are not orphans -- they are the files a real run would
+        # move -- and reporting them for deletion would misrepresent the pass
+        # twice over.
+        expected |= {
+            mirror_path_for(source, source_root, mirror_root) for source in work.values()
+        }
+    removed = prune(mirror_root, expected, dry_run) if do_prune else 0
 
     for failure in failures:
         logger.error("failed: %s: %s", failure.path, failure.error)
 
     logger.info(
-        "pass complete in %.1fs: %d encoded, %d copied, %d up to date, %d removed, %d failed",
+        "pass complete in %.1fs: %d encoded, %d copied, %d renamed, %d up to date,"
+        " %d removed, %d failed",
         time.monotonic() - started,
         counts["encoded"],
         counts["copied"],
+        counts["renamed"],
         counts["skipped"],
         removed,
         counts["failed"],