Keep attnres on resume, fix final chunk_index, default 0.5b to Yi-6B

CLI default attnres=off was overwriting block checkpoints on resume
so later loads hit Unexpected key(s). Only apply flags the user
passed. Track next_chunk so a budget-exit save does not skip the
untrained yield. 0.5b now uses 01-ai/Yi-6B (64k); refuse resume
when the ckpt tokenizer does not match.

tests/integration/test_k3_arch.py

@@ -78,6 +78,7 @@ def test_preset_0_5b_schedule():
     assert cfg.hidden_size == 768
     assert cfg.num_heads * cfg.head_dim == cfg.hidden_size
     assert cfg.num_hidden_layers == 24
+    assert cfg.vocab_size == 64000
     assert cfg.tie_word_embeddings
     assert cfg.chunk_size == 64
     assert cfg.gradient_checkpointing is True