Sprint 5L: PDF artifact parsing v0

[samrusani]

Summary

• add narrow PDF ingestion on the existing artifact-ingestion seam
• keep retrieval and compile contracts unchanged while persisting deterministic chunk rows
• add unit and integration coverage and update build/review reports

Verification

• ./.venv/bin/python -m pytest tests/unit/test_artifacts.py tests/unit/test_artifacts_main.py
• ./.venv/bin/python -m pytest tests/integration/test_task_artifacts_api.py
• ./.venv/bin/python -m pytest tests/unit
• ./.venv/bin/python -m pytest tests/integration

Review

• REVIEW_REPORT.md verdict: PASS
• Merge authorized in chat