Add open-source circuit-tracer CLT loading for CLT-Forge visualization

[HowardHsuuu]

Summary

This PR adds support for loading open-source circuit-tracer CLTs into CLT-Forge attribution workflows and visualizing the resulting graphs with the existing CLT-Forge visual interface.

It supports:

• loading circuit-tracer CLTs from HuggingFace
• loading local circuit-tracer safetensors CLTs
• loading circuit-tracer CLTs from the circuit-tracer cache
• converting circuit-tracer attribution graphs into the CLT-Forge frontend .pt graph schema
• converting circuit-tracer feature metadata into the CLT-Forge frontend feature JSON layout
• an end-to-end notebook using mntss/clt-gemma-2-2b-426k with google/gemma-2-2b

The visual interface itself is unchanged; the bridge is implemented in the library layer.

Notes

The existing CLT-Forge visual interface currently seems to hardcode the header label as GPT-2 Small; this PR does not change that UI behavior. The converted graph metadata records the actual model source, e.g. google/gemma-2-2b.

Tests

Added focused tests in tests/attribution/test_circuit_tracer_bridge.py.

These cover:

• circuit-tracer graph conversion into the CLT-Forge frontend loader contract
• selected feature indexing through active_features[selected_features]
• (layer, pos, feature_idx) to (pos, layer, feature_idx) conversion
• circuit-tracer feature metadata conversion into CLT-Forge feature JSON
• binary feature record loading from features/index.json.gz + layer_*.bin
• HuggingFace CLT loader dispatch through the circuit-tracer import alias

GPU smoke test on H100:

• loaded mntss/clt-gemma-2-2b-426k
• loaded google/gemma-2-2b
• ran attribution for The capital of France is
• generated a CLT-Forge-compatible attribution graph
• converted 50 circuit-tracer feature metadata records
• loaded the graph through the existing CLT-Forge DataLoader
• started the existing Dash visual interface successfully
• captured a rendered frontend screenshot