Fix: stop forcing matplotlib Agg backend on import (closes #2)

[Par-python]

Closes #2.

_core.py called matplotlib.use("Agg") at import time, and every measure
imports _core — so from entroscope import shannon silently switched the
user's whole matplotlib backend, breaking interactive plotting like
df["feature"].plot() in notebooks (reported by @StatisticianOne).

Fix

• Remove the forced backend from _core.py; let matplotlib resolve its own
  (it already falls back to a non-interactive backend when there's no display).
• The library no longer mutates global matplotlib state on import.

Headless safety moved to the environment

• tests/conftest.py forces Agg for the test session.
• CI test job sets MPLBACKEND: Agg.
• README documents MPLBACKEND=Agg (and ENV MPLBACKEND=Agg for Docker) for
  anyone who wants a guaranteed non-interactive backend.

I went slightly further than the suggested "move it into utils.plot", since
that would still flip the backend globally on every plot call. Nothing in the
library touches the global backend now.

Regression guard

tests/test_backend.py imports entroscope in a subprocess under a non-Agg
backend and asserts it stays put (a subprocess is needed because conftest forces
Agg session-wide). This locks the fix in.

Verification

147 passed, coverage 98.74%, ruff check + format clean. Confirmed the reported
scenario: MPLBACKEND=template ... import shannon leaves the backend as
template, not agg.