306 Repository-derived curriculum hierarchy and YAML-backed homework units

[alexeygrigorev]

Goal

Make the physical course repository the shared source of truth for nested and mixed curriculum structure, including YAML-backed homework units with Markdown prose companions.

Changed

• Parse and validate recursive module trees with one ordered sibling sequence for direct units and child modules.
• Parse structured homework units from homework.yaml plus exactly one homework.md, carrying stable unit/question identity, form fields, dates, scoring metadata, bonus/section metadata, and project references through the shared graph.
• Import/reparent existing unit and coursework rows in place, preserving learner progress, answers, and submissions; keep authored scoring answers out of learner-facing projections.
• Add recursive ordered curriculum projections, update shared rendering, format documentation, and mixed-tree/import identity regressions.
• Bump package metadata and lockfile to 0.5.13.

Verification

• uv sync --all-extras -> completed; full extras installed.
• uv run pytest -> 2,249 passed, 2 existing collection warnings.
• uv run ruff check . -> passed.
• uv run ruff format --check . -> 848 files already formatted.
• uv run python testproject/manage.py check -> no issues.
• uv run python testproject/manage.py makemigrations --check --dry-run -> no changes detected.
• Fresh SQLite migrate -> all migrations applied successfully.
• uv run pytest tests/test_boundaries.py -> 1 passed.
• uv run python scripts/plan.py check -> 172 issues, STATUS consistent.

Done when

• A mixed fixture preserves physical parentage and one authored order across direct Markdown units, YAML homework, and nested modules.
• Reparenting a YAML homework unit with the same content_id updates the existing row and preserves question, answer, submission, and progress identity.
• Homework prose comes from Markdown; structured values come from YAML; correctness is imported to the scoring field and absent from learner projections.
• Invalid manifests, companions, identities, sibling orders, and project references are rejected with actionable diagnostics; valid mixed siblings are accepted.
• Recursive module, bonus, syllabus-section, and project metadata is carried through shared projections and rendering, including one-unit modules.
• AISL and DTC consumer suites pass against this package revision.

Not run here, needs

• AISL and DTC cross-repo CI jobs are triggered by this pull request; their on-call verdict is pending.
• No package tag or release is created in this change.