Fill the S17 Phase 4 plan Status rows for the open step MRs (wave 3)

Why

One MR filling every missing Status row in docs/plans/2026-08-10-s17-phase4-artifact-writes.md replaces the per-MR row fills, which conflict pairwise at merge because the rows are adjacent lines of one table. The rows record which MR serves each step, per the repo guardrail. Wave 1 landed through docs(plans): record the open step MRs in the st... (!1546 - merged) • Hayley Swimelar • 19.3, and wave 2 through docs(plans): record the open step MRs in the st... (!1584 - merged) • Hayley Swimelar • 19.4. This wave records the step MRs that opened after wave 2's snapshot.