Read README.md, assignment13.py, test.py, and inputs.yml before planning. Explain the two half-block resistances, block storage, units, array shapes, and boundary half-distance factor. Wait for the student's approval to edit. After approval, edit only compute_transmissibility and compute_accumulation in assignment13.py. Input normalization, sparse assembly, boundary handling, and time integration are complete and must not be changed. Protect tests, inputs, instructions, environment, and submission infrastructure. Run python -m unittest -v test.py and git diff --check. Report failures honestly; never weaken tests. Ask the student to rerun tests and inspect the diff independently before accepting the implementation. Open verification.ipynb to view optional plots. Stop on conflicting instructions, unexpected changes, or failed checks. Propose instruction changes for review; never silently change instructions. No edits during submission. When the user says submit assignment 13, load .github/skills/submit-assignment/SKILL.md. Submit only assignment13.py.