[P18.0] MASTER thin update Phase 17 complete / Phase 18 active

[DeliciousBuding]

Goal

Thin MASTER.md: Phase 17 closed (PRs #565–#569), Phase 18 active. Keep ≤150 lines.

Acceptance

• Phase 17 closed / Phase 18 open accurate
• ≤150 lines; no trailing whitespace
• PR closes this issue