Background
is_prepared_fast_truncate is currently passed as false with FIXME markers in __wti_rec_row_int, __wti_rec_col_int, and __wt_cell_pack_internal_key_addr. The flag needs to be computed at the reconciliation level before page_del state can change, and the decision of whether and how to write a prepared fast-truncate cell must account for the transaction's current state relative to rec_pinned_stable_ts.

Work

Replace the false/FIXME placeholders with the correct is_prepared_fast_truncate computation in __wti_rec_row_int and __wti_rec_col_int, and implement the following selection logic for WT_PAGE_DELETED records:

Rollback case (page_del was prepared and subsequently aborted):

If rollback_ts <= rec_pinned_stable_ts: do not write the page del
Else if prepare_ts <= rec_pinned_stable_ts: write as prepared
Otherwise: do not write the page del

Commit case (page_del was prepared and subsequently committed):

If durable_ts <= rec_pinned_stable_ts: write as committed page del
Else if prepare_ts <= rec_pinned_stable_ts: write as prepared
Otherwise: do not write the page del

Prepare case (page_del is still in-flight prepared):

If prepare_ts <= rec_pinned_stable_ts: write as prepared
Otherwise: do not write the page del

Acceptance Criteria

No FIXME placeholders remain in the fast-truncate cell-building path
The three cases above are covered with tests
Existing prepare and fast-truncate tests pass with no regression