Add torch, mamba-ssm, causal-conv1d to omnirepo Python nix deps

Problem

Building a research env (~/ava/research/moral-agent) requires torch (CUDA), mamba-ssm, and causal-conv1d. These aren't in the omnirepo's Python deps list, so every new shell.nix that needs them triggers a full recompilation (~hours) instead of hitting the nix cache.

What to do

1. Add torch to Omni/Bild/Deps/Python.nix — the CUDA-enabled torch is already configured via cudaSupport = true in nixpkgs config, it just needs to be in the explicit deps list so it gets built and cached. 2. Create nix derivations for mamba-ssm and causal-conv1d (no existing derivations in the repo). Add them to the Python deps list too. 3. After merging, do a build so these get pushed to the S3 nix cache for future use.

Context

• cudaSupport = true is already set in the nixpkgs overlay
• Custom Python packages live in Omni/Bild/Deps/Python.nix and Omni/Bild/Nix/Python/
• Packages like bitsandbytes, lion-pytorch already have custom nix derivations as examples
• mamba-ssm depends on causal-conv1d, both need CUDA at build time

Acceptance criteria

• nix-shell in the omnirepo provides torch (CUDA-enabled), mamba-ssm, and causal-conv1d
• Subsequent builds can substitute from cache (no recompilation)