Document decoder residual paths and dead RoPE values

[Copilot]

Documents the decoder graph topology, including producer-consumer edges, residual branches, merge points, and fused-GQA dead-value handling.

• Residual paths

  • Traces both pre-norm attention and MLP residual branches.
  • Identifies the two Add merge points validated by the existing structural test.

• Dead-value analysis

  • Explains why discarded RoPE Gather outputs are dead.
  • Shows how direct cos_cache and sin_cache consumption avoids unnecessary graph nodes.

hidden → RMSNorm → Attention ─┐
   └──────── residual ────────┴→ Add
                               ↓
          RMSNorm → MLP ──────┐
   └──────── residual ────────┴→ Add → output

No Mobius source files are modified; the deliverable is the issue graph trace.

• Fixes ONNX for TensorRT — Day 5: Topological Graph Tracing, Residual Paths, and Dead Values #7