feat(artifacts): fetch the artifacts a protocol pins

[junwen94]

Problem

This package can upload an artifact to PSDI and verify one already on disk. It
cannot fetch one.

goldilocks_ml.artifacts.resolve builds the path a pinned dependency should
occupy, and if the file is not there it raises, naming the record and telling
the reader to go and get it:

local_data/artifacts/ptc95-vbq12/atom_init.json is missing. The
metallicity_checkpoint feature dependency is file atom_init.json from PSDI
record ptc95-vbq12 (https://data-collections.psdi.ac.uk/records/ptc95-vbq12).
Download it to that path.

Every protocol that pins an artifact therefore begins with a manual download
into a path the user has to construct correctly. The QRF95 protocol pins two;
the metallicity protocol pins one. A new user following the training guide hits
this before their first run.

The digests are already in the protocol, so the fetch has everything it needs
to be safe — it is simply not implemented.

Proposed approach

Give resolve a way to obtain what it is currently only able to describe.

• fetch a pinned dependency from its PSDI record into the artifact directory
  when it is absent, and verify the digest the protocol pins before the file is
  used for anything;
• refuse to overwrite a file that is present and correct, and refuse to keep
  one that arrives wrong;
• keep the current behaviour available: an explicit offline mode still fails
  with today's message, because a run on a plane should say what is missing
  rather than hang;
• expose it as goldilocks-ml train fetch PROTOCOL, so a user can prepare
  everything a protocol needs before starting a long job.

PSDI serves record files without authentication, so this needs no token — which
keeps it well clear of the upload path and its --confirm-upload guard.

Goldilocks Core is building an asset store for the same class of problem in
stfc/goldilocks-core#168. This is deliberately smaller: a training dependency
resolver, not a shared cache, because the two have different lifetimes and
sharing one would couple the packages for little gain.

Acceptance criteria

• A protocol's pinned dependencies can be fetched with one command
• Every fetched file is digest-verified before use, and a mismatch fails
• An offline mode reports what is missing without attempting a fetch
• The training guide no longer opens with a manual download
• Tests cover fetch, cache hit, digest mismatch, and offline

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Written by an agent on behalf of Junwen Yin.