rework: lessons 1-12 — one concept per lesson (style of 13-25)

- New app/lessons_1_12.py: 12 reworked lessons, 1 concept each,
  3 theory cards (concept / deepening / typical mistake),
  choice+term+code questions with reference solutions
- content.py: import LESSONS_1_12, exams updated to new question ids
- tests: 1-12 identity/style/reference/example-exec tests (mirroring
  13-25), prereq test for lessons 4-6 updated to new ids
- docs/CURRICULUM.md: rows 1 and 3 match reworked content

pytest 42/42, ruff check+format OK, pyright 0 errors

Author: root