Make per-clip sidecar JSONs opt-in (default off)

Previously every video_target_pipeline cut wrote a <uuid>.json provenance
sidecar alongside each <uuid>.mp4. The same provenance is already in the
per-batch plan.json, so the per-clip sidecars are redundant unless a
downstream tool wants each clip self-describing in isolation.

- video_target_pipeline.py cut: new --write-sidecar flag, default off.
- run_video_pipeline.sh: new SIDECAR env var (default "no"), passes
  --write-sidecar when SIDECAR=yes.
- README + docs/analysis/video-target-preprocessing.md updated.

The 1,984 already-emitted sidecars in /mnt/x/src/vd/ct/ct_src_*/ have
been deleted (1.5 MB).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/analysis/video-target-preprocessing.md

@@ -38,10 +38,11 @@ run_video_pipeline.sh (chain driver)
    └─ report       (HTML preview)
 
    Output: <output_dir>/<source_video_stem>/<uuid>.mp4
-                                           /<uuid>.json (sidecar)
+                                           /<uuid>.json (sidecar; opt-in via
+                                                          --write-sidecar)
 ```
 
-`run_video_pipeline.sh` is parameterized via env vars (`WORK`, `INPUT_DIR`, `OUTPUT_DIR`, `FILTER_FROM`, `SKIP_PATTERN`, `MAX_DUR`, `IDENTITY`) so you can pin a particular batch without editing the script.
+`run_video_pipeline.sh` is parameterized via env vars (`WORK`, `INPUT_DIR`, `OUTPUT_DIR`, `FILTER_FROM`, `SKIP_PATTERN`, `MAX_DUR`, `IDENTITY`, `SIDECAR`) so you can pin a particular batch without editing the script. Sidecars are off by default — the per-batch `plan.json` always carries the full provenance for every clip; the `<uuid>.json` files alongside the clips are redundant and only useful if you need each clip to be self-describing in isolation.
 
 ## 3. Quality signals (matched to inswapper_128's working envelope)