Fix: write final pos back

[kunxian-xia]

Summary

Issue: we forget to write final sponge state position back in #12.

• We add a new function in DuplexChallengerVariable called observe_slice_opt which is equivalent to observe_slice but costs much less cycles.
• Test it in unit test test_multi_observe.
  cmd: RUST_LOG=info,p3_=warn cargo test --release --test recursion test_multi_observe --package openvm-native-recursion --features "metrics,openvm-circuit/stark-debug,cuda" -- --nocapture.