data: gpt2 bpe via xserv-tokenizer + TinyStories corpus + lr schedule + grad clip

New xtrain-train crate scaffold. Data pipeline reuses xserv's from-scratch
GPT-2/Qwen BPE via a path-dep (../../../xserv/crates/xserv-tokenizer, resolves
on both ~/projects and dash5 /opt/wjh/projects): Corpus::load tokenizes the
corpus into one id stream and samples fixed-length (input, target) next-token
windows (LCG-seeded, reproducible). Trims a range-downloaded file to whole
stories (<|endoftext|> boundaries).

Also the host-only training math: LrSchedule (linear warmup + cosine decay)
and global L2 grad-norm + clip scale, each with a local unit test.

Corpus: data/tinystories-valid-3mb.txt — first ~3MB of TinyStories-valid
(fetched on dash5 via hf-mirror.com; HF direct unreachable). Substitution
noted: a real TinyStories subset, not the full set.

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

crates/xtrain-train/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "xtrain-train"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+xtrain-tensor = { path = "../xtrain-tensor" }
+xtrain-autodiff = { path = "../xtrain-autodiff" }
+xtrain-model = { path = "../xtrain-model" }
+xtrain-optim = { path = "../xtrain-optim" }
+xtrain-cuda = { path = "../xtrain-cuda" }
+# Reuse xserv's from-scratch GPT-2/Qwen BPE (project decision). This relative
+# path resolves on both ~/projects (local) and /opt/wjh/projects (dash5). The
+# 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" }