docs(plans): collapse Step 16's four Status rows into one

Replaces S16's four 16-1–16-4 Status rows with a single 16 row carrying all four part MRs: !1547 (merged), !1716 (merged), !1717 (merged), and !1718 (merged). The Status table's total row count drops from 29 to 26.

Step 16's parts are an MR-split of one deliverable, not four steps: they share one plan entry, one dependency set, and one acceptance criteria list. Row 13 is the same shape and already records both of its MRs in one cell. The 16-1–16-4 ids stay where they carry meaning, in the Step 16 parts table that names each part's files and merge order.

Follow-up outside this MR: !1716 (merged)'s title marker reads 16-2/29 and needs 16/26, since 16-2 no longer names a Status row. !1717 (merged) and !1718 (merged) are Drafts without markers and pick up 16/26 when they are readied.

Plan-table bookkeeping only. No spec, code, or test changes.