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_train_k3_resume.py

@@ -0,0 +1,81 @@
+"""Resume must not clobber attnres or skip a chunk at budget exit."""
+from argparse import Namespace
+from dataclasses import asdict
+
+import pytest
+import torch
+
+from kda.models.causal_lm import CausalLM
+from kda.models.k3_config import K3Config
+from kda.training.toy import load_ckpt
+from train_k3 import _apply_cli_overrides
+
+
+def _tiny_block():
+    return K3Config(
+        hidden_size=32,
+        num_hidden_layers=4,
+        num_heads=4,
+        head_dim=8,
+        chunk_size=4,
+        vocab_size=64,
+        moe_latent_size=16,
+        moe_d_ff=16,
+        n_routed=4,
+        top_k=2,
+        n_shared=1,
+        kv_lora_rank=8,
+        q_lora_rank=16,
+        qk_nope_head_dim=8,
+        v_head_dim=8,
+        attnres="block",
+        attnres_block_size=2,
+    )
+
+
+def _cli(**kwargs):
+    base = dict(
+        attnres=None,
+        attnres_block_size=None,
+        grad_checkpoint=None,
+        moe_aux_coef=None,
+        moe_z_coef=None,
+    )
+    base.update(kwargs)
+    return Namespace(**base)
+
+
+def test_resume_without_attnres_flag_keeps_block():
+    cfg = _tiny_block()
+    _apply_cli_overrides(cfg, _cli())
+    assert cfg.attnres == "block"
+    assert cfg.attnres_block_size == 2
+
+
+def test_explicit_attnres_overrides_resume():
+    cfg = _tiny_block()
+    _apply_cli_overrides(cfg, _cli(attnres="full", attnres_block_size=1))
+    assert cfg.attnres == "full"
+    assert cfg.attnres_block_size == 1
+
+
+def test_block_state_with_off_config_cannot_load(tmp_path):
+    cfg = _tiny_block()
+    model = CausalLM(cfg)
+    payload = {"config": asdict(cfg), "model_state": model.state_dict()}
+    payload["config"]["attnres"] = "off"
+    payload["config"]["attnres_block_size"] = None
+    path = str(tmp_path / "polluted.pt")
+    torch.save(payload, path)
+    with pytest.raises(RuntimeError, match="Unexpected key"):
+        load_ckpt(path)
+
+
+def test_budget_break_does_not_skip_yielded_chunk():
+    next_chunk = 10
+    for chunk_index in (10, 11, 12):
+        tokens = 100
+        if tokens >= 100:
+            break
+        next_chunk = chunk_index + 1
+    assert next_chunk == 10