feat: add vision transfer backbones and IMF variants

docs/superpowers/specs/2026-04-06-resnet-multitoken-imf-design.md

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+# ResNet Multitoken IMF Design
+
+**Status:** user-specified architecture, treated as approved on 2026-04-06.
+
+## Goal
+Keep a standard ResNet-18 visual trunk (no AttnRes in vision), but change IMF conditioning from one concatenated multiview token per obs step into three camera-specific condition tokens per obs step.
+
+## Approved architecture
+- Vision trunk: standard `resnet18` residual network
+- Cameras: `front`, `top`, `r_vis`
+- Each camera uses its **own** ResNet-18 weights (`use_separate_rgb_encoder_per_camera=true`)
+- Each camera produces one visual token
+- For each obs step and each camera:
+  1. take that camera visual token
+  2. concatenate robot state
+  3. project to one condition token
+- IMF input should receive **3 condition tokens per obs step**, not one concatenated token
+- With `obs_horizon=2`, IMF cond sequence length becomes `2 * 3 = 6`
+- IMF head remains on the existing IMF/AttnRes implementation path
+- Vision trunk remains standard ResNet; **no AttnRes vision replacement**
+
+## Design choices
+- Extend `ResNetDiffusionBackbone` with an opt-in mode that returns per-camera tokens shaped `(B, T, num_cams, D)` instead of concatenating camera features into `(B, T, num_cams * D)`.
+- Teach `VLAAgent` to detect multi-token visual features, broadcast state per camera token, apply the existing condition projector on each token, then flatten `(T, num_cams)` into one cond sequence for the IMF head.
+- Keep `per_step_cond_dim` as the width of a single condition token, and add explicit token-count metadata so transformer heads get the correct cond-sequence length.
+- For the new experiments, set the condition-token width equal to `n_emb` via `cond_projector.output_dim=${agent.head.n_emb}`.
+
+## Files expected to change
+- `roboimi/vla/models/backbones/resnet_diffusion.py`
+- `roboimi/vla/agent.py`
+- new Hydra agent config for the multitoken ResNet IMF variant
+- focused tests in `tests/test_imf_vla_agent.py` and/or `tests/test_resnet_transformer_agent_wiring.py`