…tep (b) bisection result

Records the LIVE bisection result for M-GPU-MOE-1.4 step (b)
from operator-dispatched run on Blackwell GB10 (gx10) on 2026-05-06.

WHAT THE BISECTION FOUND:
- First NaN_GPU on `moe_ffn_out` = layer 6
- L0–L5: ALL MATCH on both moe_router AND moe_ffn_out (cos > 0.99986)
- L6: first NaN_GPU on moe_ffn_out (router still finite at L6)
- L7+: all DIVERGE on router (downstream NaN poisoning)

Decision tree firing per harness output:
  "If first_NaN_GPU(moe_ffn_out) > 0 and earlier layers MATCH:
   bug is layer-N specific (rare)."

ARCHITECTURAL PORTABILITY:
Bisection ran on sm_120 (Blackwell GB10). Original M-GPU-MOE-1.3
NaN bug (PR #1493) was characterized on sm_89 (Ada RTX 4090). Both
architectures produce NaN at the same layer → bug is algorithmic /
numerical, NOT kernel codegen. A single fix at the bisected stage
discharges both arch-specific manifestations.

BUG SURFACE NARROWED:
- crates/aprender-serve/src/gguf/cuda/moe_ffn_forward_layer_cuda.rs
- crates/aprender-serve/src/gguf/cuda/expert_swiglu_cuda.rs
- CudaExecutor::q4k_matvec / q6k_gemv

The bug is NOT in the routing logic (router stays finite at L6).
It's in the per-expert FFN computation at layer 6 specifically.

HYPOTHESES (refined from v1.4.0, priority order):
1. Numerical overflow in expert SwiGLU at L6
2. Expert weight distribution at L6 produces large activations
3. Q4K dequant accumulator at L6 overflow

The v1.4.0 hypothesis "missing per-head Q/K RMSNorm" is REFUTED —
Q/K norm runs in attention which is BEFORE FFN; if it were missing
the divergence would appear earlier than L6.

IMPLEMENTATION_STAGE M-GPU-MOE-1.4: PENDING → PARTIALLY_DISCHARGED.
- Step (a) instrumentation cascade: COMPLETE (M50→M81)
- Step (b) LIVE bisection: COMPLETE (this evidence)
- Step (c) fix: OPEN

Sibling contract `trace-moe-gpu-sub-stages-v1` v1.4.0 → v1.5.0
amendment in PR #1527 records the same bisection result from
the falsifier-side. This PR records it from the parent kernel-
contract side; both refer to the same evidence dir.

YAML-only — production hot paths byte-unchanged.

`pv validate` 0/0.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>