feat(py): accept m_eff, and export M_E

[reneotten]

feat(py): accept m_eff, and export M_E

The Python wrapper's docstring said keyword arguments "match the Rust
DeviceParams field names", but m_eff was hardcoded to 0.9 * M_E in the
binding, so Device(m_eff=...) raised TypeError. The README discusses
m_eff as a tunable fitting parameter, so the documentation actively
pointed at an argument that did not exist.

Accept m_eff as an optional keyword. The default is unchanged.

m_eff is an absolute mass in kilograms, and the natural way to write "0.9
electron masses" is m_eff=0.9, which would silently be ~10^30x too heavy.
Export M_E so the intent can be spelled m_eff=0.19 * negforge.M_E, and
reject values above 1e-25 kg with an error that shows the correct form.
Non-finite and non-positive values are rejected too.

Also documents an asymmetry found while testing this: m_eff only enters
the NEGF hopping t_hop = hbar^2 / (2 m_eff a^2), so it changes local_density_of_states()
and the self-consistent charge, but not calc_current(): the ballistic
Landauer integral in this model assumes unit transmission above the barrier
and carries no mass prefactor. Changing m_eff alone therefore leaves the
ballistic I-V untouched by construction. Pinned by a test so the behaviour
is a stated property rather than a surprise.

[Copilot]

Pull request overview

This PR updates the Python bindings to fully support m_eff as a configurable Device parameter (instead of being hardcoded), and exposes the free-electron mass constant M_E so Python callers can specify effective masses safely in SI units.

Changes:

• Add optional m_eff keyword argument to the PyO3 Device constructor, preserving the existing default.
• Export M_E to Python and re-export it from negforge for ergonomic usage (negforge.M_E).
• Add a core Rust test that pins the intended behavior: m_eff scales t_hop but does not affect the ballistic calc_current() result.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
python/negforge/init.py	Re-exports M_E and documents correct m_eff units/usage and its modeled impact.
crates/negforge-py/src/lib.rs	Accepts m_eff=None, validates units/range, and exports M_E from the extension module.
crates/negforge-core/src/device.rs	Adds a regression test asserting m_eff affects hopping but not ballistic current.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.