Port `_calculate_new_extremities`, `_calculate_new_state_dag_extremities`
and `_prune_extremities` from the persist events controller to
`rust/src/persist/extremities.rs`. They read through an `ExtremityReads`
trait, which the `PersistReader` implements and the unit tests implement
over an in-memory DAG. Metrics go to a `PersistCounts` the caller owns,
so shadow mode can drop them.
Missing prev state events fail with `MissingPrevStateEvents`, which
`persist_error_to_py` turns into the same 500 `SynapseError` as Python.
`PersistReaderForTest` gains the three calculations, and
`test_persist_reader.py` compares each with the Python method on the
same database and the same new events.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UHKYPSc37kWqGACn9aKxKL