[P17.0] MASTER thin update Phase 16 complete / Phase 17 active

[DeliciousBuding]

Goal

Thin MASTER.md: Phase 16 closed (PRs #554–#558), Phase 17 active. Keep ≤150 lines.

Acceptance

• Phase 16 closed / Phase 17 open accurate
• ≤150 lines; no trailing whitespace
• PR closes this issue