ci: add missing test_site_reference_snapshot.py to a pytest shard

[rizumita]

Summary

• The 3-way pytest shard split introduced in ci: split pr-full pytest run into 3 balanced shards #164 (ci: split pr-full pytest run into 3 balanced shards) omitted tests/test_site_reference_snapshot.py from .github/ci-shards/shard-*.txt.
• The new shard coverage check job (.github/workflows/*.yml, pr-shard-coverage) diffs the union of the shard files against tests/test_*.py and fails if they don't match exactly — so this currently fails on every PR against main, independent of what the PR changes. Discovered while investigating an unrelated docs PR (docs(site): deepen the fsl-domain and fsl-ai manual pages #165) whose only failing check was this one.

Changes

• .github/ci-shards/shard-1.txt — add tests/test_site_reference_snapshot.py (alphabetically placed). It's a fast file-diff/freshness test (~0.1s), so it doesn't materially change shard-1's runtime balance.

Verification

cat .github/ci-shards/shard-*.txt | sort > /tmp/covered.txt
ls tests/test_*.py | sort > /tmp/actual.txt
diff -u /tmp/actual.txt /tmp/covered.txt   # empty diff now
pytest -q tests/test_site_reference_snapshot.py   # 2 passed

Risk / Review Notes

• CI-only change, no test/source semantics touched. Zero risk to shard runtime balance given the test's cost.

Follow-Up / Post-Merge Checks

• None.