Document Pyodide compatibility pattern

TEMPLATE.md

@@ -42,7 +42,42 @@ repo, removes the template banner from `README.md`, and finally deletes
   PathSim core example; replace it with notebooks that demonstrate your
   blocks. Every `*.ipynb` here is executed and rendered by the docs build.
 
-## 4. Register with the docs build
+## 4. Stay compatible with the web runtime
+
+PathView ships a browser build that runs Python through Pyodide. Pure-Python
+toolboxes load there out of the box, but dependencies that ship native code
+(`pybamm`, `jsbsim`, `casadi`, anything that needs a compiled wheel) fail to
+install in the browser.
+
+Default `pip install pathsim-<name>` on Linux/macOS/Windows should still
+install everything — no extras required. To get there:
+
+1. **Mark heavy dependencies in `pyproject.toml`** with a PEP 508 environment
+   marker so micropip (Pyodide) silently skips them:
+
+   ```toml
+   dependencies = [
+       "pathsim>=0.22",
+       "numpy>=1.15",
+       "pybamm>=25.12; sys_platform != 'emscripten'",
+   ]
+   ```
+
+2. **Guard the re-export in `src/pathsim_<name>/__init__.py`** so the rest of
+   the toolbox stays importable when the heavy dep is missing:
+
+   ```python
+   try:
+       from .cells import CellElectrical
+       __all__ += ["CellElectrical"]
+   except ImportError:
+       pass
+   ```
+
+In PathView's web Toolbox Manager, introspection then finds only the blocks
+whose submodules imported successfully; the desktop install sees everything.
+
+## 5. Register with the docs build
 
 The documentation site (`docs.pathsim.org`) is built by the
 [`pathsim/docs`](https://github.com/pathsim/docs) repository. It clones every
@@ -70,7 +105,7 @@ The repo-side docs contract is just: an importable package under `src/` and
 `*.ipynb` files under `docs/source/examples/`. No Sphinx — the docs build owns
 all rendering.
 
-## 5. Releasing
+## 6. Releasing
 
 `.github/workflows/publish.yml` publishes to PyPI via trusted publishing when
 a GitHub Release is published. Versions come from git tags via `setuptools-scm`,

pyproject.toml

@@ -27,6 +27,17 @@ classifiers = [
 dependencies = [
     "pathsim>=0.22",
     "numpy>=1.15",
+    # If your toolbox needs a dependency that ships native code (e.g.
+    # pybamm, jsbsim, casadi, …) it will fail to install in Pyodide. Tag
+    # it with a PEP 508 environment marker so a normal pip install on
+    # Linux/macOS/Windows still pulls it eagerly, but micropip in the
+    # browser silently skips it. Example:
+    #
+    #     "pybamm>=25.12; sys_platform != 'emscripten'",
+    #
+    # Pair this with a defensive `try/except ImportError` re-export in
+    # `src/pathsim_<name>/__init__.py` so the rest of the toolbox stays
+    # importable when the heavy dependency is absent.
 ]
 
 [project.optional-dependencies]

src/pathsim_toolbox/__init__.py

@@ -11,6 +11,19 @@
 except ImportError:
     __version__ = "unknown"
 
+# Pure-Python blocks — eager.
 from .example_block import FirstOrderLag
 
 __all__ = ["__version__", "FirstOrderLag"]
+
+# If you add blocks that depend on a native package (e.g. pybamm, jsbsim,
+# casadi, …) put them in their own submodule and re-export them defensively
+# here so the toolbox still imports when the heavy dependency is missing
+# (e.g. in Pyodide). Pair this with a PEP 508 environment marker in
+# pyproject.toml — see the comment there for an example.
+#
+# try:
+#     from .native_block import HeavyBlock
+#     __all__ += ["HeavyBlock"]
+# except ImportError:
+#     pass