curriculum: materialize readable solutions for variables and strings

[abhiksark]

Motivation

The current solution files are loader stubs into _answers.py. Learners and reviewers benefit from direct, readable reference code they can inspect one exercise at a time.

Current evidence

• solutions/variables1.py through variables10.py and solutions/strings1.py through strings10.py are part of the loader-stub migration target.
• Reference solutions are verified by tests/integration/test_solution_verify.py.
• The project supports Python 3.9+.

Exact scope

Replace the 20 loader stubs for variables and strings with direct teaching-quality Python source. Preserve each expected answer and keep _answers.py entries intact until the full 292-file migration is complete.

Acceptance checklist

• solutions/variables1.py through variables10.py contain direct readable source, not loader imports or delegation.
• solutions/strings1.py through strings10.py contain direct readable source, not loader imports or delegation.
• Each solution preserves the behavior expected by its matching hidden check.
• Source favors clear beginner teaching over clever compression and is Python 3.9 compatible.
• The corresponding _answers.py entries remain unchanged for the wider staged migration.

Explicit exclusions

Do not change exercises, checks, manifest ordering, unrelated solution slices, or remove/restructure _answers.py.

Likely files

• solutions/variables1.py … solutions/variables10.py
• solutions/strings1.py … solutions/strings10.py

Verification

python -m pytest tests/integration/test_solution_verify.py -q
pythonlings --root tests/fixtures/passing_curriculum verify
python -m pytest -q

Also manually inspect all 20 files to ensure they are direct teaching-quality Python source.

Sprint coordination

See the sprint Discussion. This issue is unassigned and is not a reservation.