Function select_thread_base 

pub fn select_thread_base(
    requested_from: Option<StateId>,
    existing_tip: Option<StateId>,
) -> Result<ThreadBaseSelection, ThreadBaseError>

Select the base state for a new or resumed thread start.

Rules (matching CLI start_thread):

1. Existing tip + matching --from (or no --from) → use existing tip
2. Existing tip + mismatched --from → ThreadBaseError::AnchorMismatch
3. No existing tip + --from → use the resolved --from
4. Neither → ThreadBaseSelection::RequireCurrent