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>

crates/xtrain-train/Cargo.toml

@@ -14,7 +14,14 @@ xtrain-cuda = { path = "../xtrain-cuda" }
 # crate inherits xserv's workspace for its own deps (serde/regex) — Cargo reads
 # the target package's workspace, not ours.
 xserv-tokenizer = { path = "../../../xserv/crates/xserv-tokenizer" }
+# T9 export to xserv: HF Qwen3 safetensors + BF16 weight cast.
+half.workspace = true
+safetensors = "0.5"
 
 [[bin]]
 name = "train"
 path = "src/bin/train.rs"
+
+[[bin]]
+name = "export_safetensors"
+path = "src/bin/export_safetensors.rs"