feat: manual exports use vid number with m{N} tag

Manual clips now follow the same pattern as scan exports:
clip_003_m1_0.mp4 (manual) vs clip_003_a1_0.mp4 (auto-scan).
The clip number matches the vid folder number.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/paths.py

@@ -24,17 +24,23 @@ def _log(*args) -> None:
     print(f"[8-cut {ts}]", *args, file=sys.stderr)
 
 
-def build_export_path(folder: str, basename: str, counter: int, sub: int | None = None) -> str:
+def build_export_path(folder: str, basename: str, counter: int,
+                      sub: int | None = None, tag: str | None = None) -> str:
     """Build clip output path.  *folder* should be the vid folder (e.g. .../mp4/vid_001)."""
     name = f"{basename}_{counter:03d}"
+    if tag is not None:
+        name = f"{name}_{tag}"
     if sub is not None:
         name = f"{name}_{sub}"
     return os.path.join(folder, name + ".mp4")
 
 
-def build_sequence_dir(folder: str, basename: str, counter: int, sub: int | None = None) -> str:
+def build_sequence_dir(folder: str, basename: str, counter: int,
+                       sub: int | None = None, tag: str | None = None) -> str:
     """Build WebP sequence output dir.  *folder* should be the vid folder."""
     name = f"{basename}_{counter:03d}"
+    if tag is not None:
+        name = f"{name}_{tag}"
     if sub is not None:
         name = f"{name}_{sub}"
     return os.path.join(folder, name)