Cover both astrolabe faces in foundations

[jbeshir]

Summary

• remove the incorrect claim that the initial reference-position rete is set for a date and time
• expand the foundations lesson to introduce the fixed back scales and independently moving alidade
• retitle the lesson to cover the whole astrolabe rather than only the front
• make checkpoint-gated Next buttons explicitly and visibly disabled until Check passes

Validation

• npm test (241 tests)
• npm run build
• node ../../scripts/validate-widgets.mjs
• browser journey: 60 viewport/theme cells