feat: add vision transfer backbones and IMF variants

docs/superpowers/specs/2026-04-06-siglip2-multiview-vla-design.md

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+# SigLIP2 Multiview VLA Design
+
+**Status:** user-specified architecture, treated as approved on 2026-04-06
+
+## Goal
+Replace the current vision encoder for the IMF/AttnRes diffusion policy with a frozen SigLIP2 image encoder while preserving the downstream action-diffusion stack and rollout behavior.
+
+## Approved architecture
+- Backbone model: `google/siglip2-base-patch16-256`
+- Camera inputs: three views, encoded **independently** with a **shared** SigLIP2 vision encoder
+- Input size:
+  - dataset images stay at native `256x256` (no dataset-side resize)
+  - eval/rollout images resize to `256x256` before SigLIP2 because env renders are larger
+- Per-view feature: use the global pooled image feature (`pooler_output`, 768-d)
+- Per-view projection experiments:
+  1. `768 -> 96`
+  2. `768 -> 192`
+- Conditioning pipeline:
+  1. concatenate 3 projected camera vectors
+  2. concatenate robot state
+  3. project concatenated condition to `384`
+  4. feed that `384`-d per-step condition into the existing IMF/AttnRes diffusion head
+- Training/run defaults for requested experiments:
+  - `n_emb=384`
+  - `n_layer=12`
+  - `pred_horizon=16`
+  - `num_action_steps=8`
+  - rollout count for validation: keep current requested behavior on this branch unless explicitly overridden later
+
+## Design decisions
+- The condition projector lives in `VLAAgent._build_cond()` so the backbone owns only visual features, while the agent owns the final conditioning contract expected by the diffusion head.
+- The SigLIP2 backbone is frozen by default; only the per-view projectors and downstream policy layers train.
+- The backbone exposes `dataset_image_resize_shape=None` and `eval_image_resize_shape=(256, 256)` so existing train/eval plumbing can reuse the raw-256 path already added in this branch.
+- One shared vision encoder is used across cameras to keep memory and download size reasonable and to match the user's request for per-view independent encoding rather than a fused multiview image.
+
+## Files expected to change
+- `roboimi/vla/models/backbones/` for the new SigLIP2 backbone
+- `roboimi/vla/agent.py` for optional post-concat condition projection
+- Hydra configs under `roboimi/vla/conf/{agent,backbone,modules}`
+- tests for backbone wiring and agent conditioning dims
+- remote launch commands/scripts only as needed for training