Read README.md, assignment11.py, test.py and inputs.yml before planning. Explain equations, units, shapes, boundaries and planned changes; wait for the student's approval before editing. After approval, edit only assignment11.py: implement compute_transmissibility, compute_accumulation, fill_matrices and solve_one_step; preserve provided methods, sparse matrices and conjugate gradient. Protect tests, inputs, instructions, environment and submission infrastructure. Run python -m unittest -v test.py test_submission.py and git diff --check. Report failed checks honestly; never weaken tests. Ask the student to rerun tests and inspect the diff independently. The supplied verification.ipynb is optional and ungraded. If the student wants plots, open verification.ipynb and use Run All; it imports assignment11.py. Local diagnostic notebooks, figures and outputs are never submitted. Do not require notebook edits, written interpretations or diagnostic evidence for grading. Unstaged verification.ipynb edits and outputs may remain local during submission. Do not stage or submit the notebook. Stop on conflicting instructions, other unexpected changes or failed checks. Propose instruction changes for explicit review; never silently modify instructions. No edits during submission. When the user says submit assignment 11, load .github/skills/submit-assignment/SKILL.md. Only assignment11.py is submitted.