Move `_find_shard_with_sid`, `_resolve_shard`, `_validate_names`,
`_get_index`, and the index cache from
`decnet/web/router/transcripts/api_get_transcript.py` into
`decnet/artifacts/shards.py`. The shared module speaks
`ValueError`; the router keeps thin wrappers that translate to
`HTTPException(400)` so the route's error UX is unchanged.
This unblocks the BEHAVE-INTEGRATION Phase 4 worker wiring — the
profiler worker (and the collector's session aggregator) need to
disk-reach asciinema shards but must not import from a FastAPI
router.
11 new unit tests for the shared helper. Existing transcript router
tests pass (the shard fixture's monkeypatch points at the shared
module's ARTIFACTS_ROOT now).
anti
588ea4e411