feat: causal diff engine — reverse-engineer reasoning from weight-space (#56)

* feat: causal_diff engine — NARS-truthed weight-space diffing

Diff two bgz7 indexes by tensor name + row, emit CausalEdges
with NARS truth values. Classifies edges by attention projection
(Q/K/V/O/Gate/FFN), clusters by block, finds reasoning scaffold
circuits (Q+O shift, K stable).

Includes NARS revision across multiple diff runs for integrated
evidence about structural reasoning patterns.

Test: test_full_reasoning_reverse_eng indexes 5 Qwen3.5 Q8_0
models (105 GB), diffs 4 pairs, revises across all.

* mod: register causal_diff module

* feat: CompressedTensor::read_from + read_bgz7_file

Deserializer for bgz7 files — reads back the per-tensor Base17
rows written by stream_index_gguf/stream_index_gguf_bf16.
Required by causal_diff to load and compare indexed models.

Author: AdaWorldAPI