[M0-S7] Repair vertical seams in stage_art.py #62

Problem

Chunks will grow up and down (a scaffold tower, a culvert). Only horizontal seams are repaired today.

Acceptance criteria

• Up/down join windows repaired like left/right ones, protected pixels verified
• Tests in scripts/test_stage_art.py

Notes and dependencies

See 09 – Growing a stage. Not needed for the jam slice; first to cut.

Verification

Unit tests; a test chunk pair joins without a visible seam.