Function local_recovery_batch_step 

pub fn local_recovery_batch_step(
    position: u64,
    file_end: u64,
    expected_offset: i64,
    batch_base: i64,
    last_offset_delta: i32,
    encoded_len: u64,
) -> Option<LocalRecoveryStep>

Admit one completely decoded batch into the maximal valid tail prefix.

Compaction gaps are allowed, but overlap, empty byte progress, file-bound overflow, malformed offsets, and signed offset overflow stop the prefix.