Add SCAIL-2 in-context mask channels (Phase 2, plumbing + zero-init surgery)

Port mechanism 2 of SCAIL-2 (arXiv:2606.10804) to LTX-2: extra per-token
in-context conditioning channels (1 environment switch + K=6 character binding
slots) concatenated onto the latent before the first projection.

Faithful temporal encoding for LTX's VAE (temporal factor 8): each latent frame
stacks its 8 pixel sub-frames along the channel dim, giving 8*(K+1)=56 channels
(vs the paper's 4*(K+1)=28 on Wan 2.1).

Plumbing (backward compatible; cond_channels=None leaves every existing pipeline
unchanged):
- LatentState/Modality gain an optional cond_channels field (patchified [B,T,C]).
- LTXModel(mask_conditioning_channels=0) config-gates a widened patchify_proj;
  TransformerArgsPreprocessor concatenates cond_channels (or zero-pads) before it.
- tools.clear_conditioning trims it; token-appending conditioning items
  (reference video/audio, driving, keyframe) extend it via extend_cond_channels.

New:
- VideoConditionByMaskChannels: encodes (K+1) pixel masks -> 56 channels, written
  onto the trailing driving tokens (noisy target stays all-zero, per the paper).
- widen_patchify_proj_for_mask_channels: zero-init checkpoint surgery so a
  converted model reproduces the base output exactly until finetuned.

Verified (CPU, random weights): backward compat, zero-init widened forward is
bit-identical to baseline for any cond_channels, and the driving+mask pipeline
forwards without crashing with correct placement/clipping. Visual quality
requires Phase 3 finetuning; Replacement-mode z_ref height shift still deferred.

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

packages/ltx-pipelines/src/ltx_pipelines/utils/helpers.py

@@ -269,6 +269,7 @@ def modality_from_latent_state(
         context=context,
         context_mask=None,
         attention_mask=state.attention_mask,
+        cond_channels=state.cond_channels,
     )