Add SCAIL-2 animation inference pipeline (Phase 4, ltx-pipelines)

Two-stage distilled inference pipeline that animates a character from a driving
video, wiring the Phase 1-3 SCAIL-2 conditioning into a runnable CLI.

- scail_animation.py: ScailAnimationPipeline (mirrors distilled.py) plus a pure,
  testable build_scail_conditionings that assembles VideoConditionByDrivingLatent
  + VideoConditionByMaskChannels (driving appended, mask on the trailing driving
  tokens), and a load_masks helper. main() + scail_animation_arg_parser add
  --driving-video / --mask-path / --mode / --driving-strength on top of the
  standard two-stage distilled parser.
- LTXModelConfigurator now reads config `mask_conditioning_channels`, so a
  SCAIL-trained checkpoint whose config declares it builds the widened
  patchify_proj automatically — no runtime widening wrapper needed at inference.
- CLAUDE.md pipeline table row.

Verified on CPU (verify_phase4_pipeline.py): the module imports, the CLI parses
the SCAIL flags, build_scail_conditionings grows the sequence and places the mask
channels on the driving tokens (target stays zero), driving-only leaves
cond_channels None, and the configurator honors mask_conditioning_channels
(patchify_proj widened from config). End-to-end runs still need a GPU and a
SCAIL-trained checkpoint.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/ltx-core/src/ltx_core/model/transformer/model_configurator.py

@@ -69,6 +69,7 @@ class LTXModelConfigurator(ModelConfigurator[LTXModel]):
             caption_projection=caption_projection,
             audio_caption_projection=audio_caption_projection,
             cross_attention_adaln=config.get("cross_attention_adaln", False),
+            mask_conditioning_channels=config.get("mask_conditioning_channels", 0),
         )
 
 
@@ -120,6 +121,7 @@ class LTXVideoOnlyModelConfigurator(ModelConfigurator[LTXModel]):
             apply_gated_attention=config.get("apply_gated_attention", False),
             caption_projection=caption_projection,
             cross_attention_adaln=config.get("cross_attention_adaln", False),
+            mask_conditioning_channels=config.get("mask_conditioning_channels", 0),
         )