npu-pack: a pack with no text artifacts imports clean and only fails later, at 1bit serve

What happens

1bit npu-pack import copies the pack's tokenizer/config files when it finds them and says nothing when it
finds none. The result is a model directory with model.q4nx + config.json and no tokenizer.json, and the
command that then fails is 1bit serve, two commands later:

$ 1bit serve -m <imported dir>
<dir> is not an NPU model directory (model.q4nx, config.json, tokenizer.json)

That message reads like an engine bug, not an incomplete pack, and there is nothing in the import's own output to
point at the pack.

Why it matters beyond a missing file

Two of the four AMD OGA packs on this box were exactly that case — Qwen3-1.7B_rai_1.8.0_npu_4K and
Qwen3-4B_rai_1.8.0_npu_4K held 5 files each (weights + config.json + cache/) because they were fetched weight
by weight. AMD's repos ship all of it: 71 files each, including tokenizer.json (11,422,654 B),
tokenizer_config.json, vocab.json, merges.txt, chat_template.jinja, genai_config.json. The silence at
import time is what let a previous session conclude "the Qwen3 packs ship no tokenizer files" — a false statement
about the vendor's packs recorded as a fact (amd-oss/npu-re FACTS §7.26).

Which text files a pack carries also varies by pack, so a fixed expectation is wrong in both directions: the
Llama-3.2-1B pack ships a 17.2 MB tokenizer.json and no vocab.json/merges.txt at all.

Measured (before)

1bit npu-pack info on a 5-file pack: no mention of the tokenizer, chat template or generation config anywhere in
the 200-line JSON.

Fix

Report the split instead of assuming it:

Same shape as the existing tools/gguf_to_q4nx.py behaviour ("warns loudly when tokenizer.json is missing") that
the pack path never had. servable is exactly "tokenizer.json is present", which is what docs/npu.md requires
of a model directory.

Verification

• npu_pack_import_test gains test_text_artifacts: a pack with none reports 8 missing / not servable, and the
  same fact through manifest(); with two files present it reports those two, six missing, servable.
• Real packs, both directions: a 5-file pack (warns, container records "none in the pack") and a complete one
  (8 of 8 ... copied, container records "copied from the pack" and 1bit serve starts).