data: full TinyStories + tokenized-id cache, val loss, CLI arch

- Corpus::load_cached: tokenize the (large) corpus ONCE, cache the id stream to
  <corpus>.u16.bin (gpt2 vocab 50257 < 65536 → exact u16), read cache on reruns.
- Corpus::split_tail: hold out a tail slice as a validation corpus.
- train(): take an optional valid corpus + eval_every/eval_batches; periodic
  deterministic val-loss eval that checkpoints the BEST val model; returns
  TrainResult{train_losses, evals, best_val}. T6 fixed-cadence path preserved.
- bin/train + bin/export_safetensors: read architecture (--heads/--head-dim/
  --layers/--ffn) + opt knobs (--steps/--batch/--seq/--max-lr/--val-tokens/
  --eval-every) from CLI flags; defaults reproduce the v0-baseline tiny config.
- gitignore the multi-GB corpus + *.u16.bin caches + *.ckpt (dash5-only).

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

.gitignore

@@ -9,3 +9,9 @@
 
 # Claude Code runtime state
 /.claude/
+
+# Large scaling-run corpora + tokenized id caches live on dash5 only, never in
+# git (the small data/tinystories-valid-3mb.txt is committed as a fixture).
+/data/tinystories-train.txt
+*.u16.bin
+*.ckpt