refactor: organize src modules by category

AGENTS.md

@@ -1,12 +1,13 @@
 # Repository Guidelines
 
 ## Project Structure & Module Organization
-Core simulation code lives in `src/`. The base pipe blowdown model is split across `src/tank.py`, `src/pipe.py`, `src/riemann.py`, `src/solver.py`, and `src/output.py`, with `src/main.py` as the entry point. The cryogenic tank variant is isolated under `src/cryo_tank/`. Tests mirror that layout in `tests/` and `tests/cryo_tank/`. Use `cases/` for scenario inputs, `docs/` for design notes and generated reports, `scripts/` for utilities, and `results/` for run artifacts.
+Core simulation code lives in `src/`. The base pipe blowdown model is grouped under `src/tank_pipe/`, the high-pressure gas cylinder model is under `src/cylinder/`, and the cryogenic tank variant is isolated under `src/cryo_tank/`. `src/main.py` remains a compatibility entry point for the tank-pipe simulation. Tests live in `tests/` and `tests/cryo_tank/`. Use `cases/` for scenario inputs, `docs/` for design notes and generated reports, `scripts/` for utilities, and `results/` for run artifacts.
 
 ## Build, Test, and Development Commands
 There is no checked-in build system; run modules directly from the repository root.
 
-- `python3 src/main.py`: run the 0D-1D tank-pipe simulation and write plots/reports to `results/`.
+- `python3 src/main.py`: run the 0D-1D tank-pipe simulation through the compatibility entry point.
+- `python3 src/tank_pipe/main.py`: run the 0D-1D tank-pipe simulation from its package entry point.
 - `python3 src/cryo_tank/main.py`: run the cryogenic LN2 tank simulation.
 - `pytest -q`: run the full test suite.
 - `pytest -q tests/test_integration.py`: run the main conservation tests only.