feat(stage-8d-7): direct safetensors fused-region loader

Replaces load_fused_qkv_slice_2d/_3d with reads from a separate
MmapedSafetensors handle. Each per-rank fused tensor is built by
reading the three region byte-slices directly from the mmap,
concatenating them host-side, and uploading as one device
allocation — no full-fused-tensor device materialisation.

The prior approach allocated a ~100 MB transient device tensor
per linear-attention layer; on Qwen3.6-27B with 48 linear-attn
layers that's ~4.8 GB of allocator churn during load — enough
to fragment the cuda caching allocator on a tight-VRAM 32 GB
consumer GPU, which is what triggered the layer-22 up_proj
OOM seen on beast.

Threading: MmapedSafetensors flows worker → ForCausalLM →
Model → DecoderLayer → GatedDeltaNet::load. Both leader (mod.rs)
and worker (worker.rs) construct their own mmap; Linux's page
cache shares the underlying pages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2120,6 +2120,7 @@ dependencies = [
  "half",
  "hf-hub",
  "reqwest",
+ "safetensors 0.7.0",
  "serde",
  "serde_json",
  "thiserror 2.0.18",