export: safetensors + config.json for xserv qwen3

New bin export_safetensors: load an xtrain checkpoint, map every param to its
HF Qwen3 tensor name, transpose 2D projection weights [in,out]->[out,in]
(1D norms + [vocab,dim] embed/lm_head kept), cast to BF16 (xserv's qwen3
forward is BF16-only), and write config.json + model.safetensors + a copy of
the gpt2 tokenizer.json. Sized exactly like bin/train.rs. safetensors 0.5 to
match xserv. GPU body gated behind not(no_cuda).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -109,6 +109,16 @@ version = "0.8.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d6f6ff9a378485b298a5286656da665ba74413d36db0979633275d2e708145d4"
 
+[[package]]
+name = "safetensors"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cc0cdb7198d738a111f6df8fef42cb175412c311d0c4ac9126ff4e550ad1a0e8"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "serde"
 version = "1.0.228"
@@ -249,6 +259,8 @@ dependencies = [
 name = "xtrain-train"
 version = "0.1.0"
 dependencies = [
+ "half",
+ "safetensors",
  "xserv-tokenizer",
  "xtrain-autodiff",
  "xtrain-cuda",