docs(snapshot): scrub stale "verifier not shipped" wording

[VARUN3WARE]

Summary

Align the dataset-snapshot how-to and the integrity exporter docstring with the shipped verifier (hflow verify snapshot / verify_dataset_snapshot). Closes #553.

Why

The export how-to introduced the integrity block as something a later verifier "(not shipped here)" would check, then documented running hflow verify snapshot a few paragraphs later. The exporter helper docstring still said "without a verifier product yet." That contradiction is leftover from before #457 / #473 / #469 landed.

Two commits:

1. Scrub the how-to intro so it names the shipped CLI/API.
2. Point _build_snapshot_integrity_marker_fields at verify_dataset_snapshot.

Validation

# No stale phrases remain in the touched sources
grep -n 'not shipped\|verifier product\|without a verifier' \
  docs/how-to/export-dataset-snapshot.md src/hflow/snapshot.py
# (expect no matches)

# How-to intro and verify section both name the shipped commands
grep -n 'hflow verify snapshot\|verify_dataset_snapshot' docs/how-to/export-dataset-snapshot.md

No behavior change; docs/docstring only. Pre-commit ruff clean on the Python touch.

Checklist

• I added or updated outcome-focused tests for changed business logic.
• I updated documentation for changed behavior, flags, formats, or requirements.
• I ran uv run ruff check --fix, uv run ruff format, and uv run ty check.
• I ran the relevant pytest suite.
• I did not add recordings, generated media, credentials, private URLs, or runtime artifacts.
• I preserved stored-data compatibility or documented an explicit version change.

[kstonekuan]

LGTM, merging.

Catching the same stale claim in snapshot.py:251 ("without a verifier product yet"), not just the line the issue named, is the difference between fixing the report and fixing the problem. Nothing matching is left in docs/ or src/.