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.

Author: AdaWorldAPI