refactor(state_store): finalize ops migration and clean up

- Replace cursor-based sorted merge in Committer::update_existence with
  ops::list_child_existence + ops::{write,delete}_child_existence.
- Type deferred_writes as a DeferredWrite enum; flush dispatches to the
  right ops::* per variant. Drop encode_owner_upsert helper.
- Use ops::read_tracking_info for ownership-preempt reads in pre_commit.
- Remove Committer.encoded_tombstone_key_prefix and the &[u8] key
  parameters on commit/commit_in_txn; derive everything from StablePath
  via ops.
- Drop speculative ops with no callers (ComponentSnapshot,
  load_component_snapshot, plus per-entity ops that ended up unused);
  demote internal-only helpers to private.
- Retype inspect helpers in state_store::ops to take &heed::Env
  directly so state_store/ has no engine:: imports.

Author: georgeh0