Fix CI test and doctest failures with updated NumPy, SciPy, and Matplotlib - #1243

A collection of small, independent fixes for test and doctest failures
appearing in CI. No functional changes to the library API. Open PRs
with failing CI tests should rebase onto main once this is merged.

NumPy: unsupported shape assignment

ndarray.shape assignment is no longer supported in these contexts;
replaced with reshape().

• control/descfcn.py — describing_function()
• control/statesp.py — StateSpace.__init__(), static-system reshaping

NumPy: complex-to-real casts

Discard the (numerically zero) imaginary parts explicitly rather than
letting NumPy warn on the implicit cast.

• control/modelsimp.py — hankel_singular_values() takes sqrt(w.real),
  since Wo @ Wc is symmetric positive semidefinite in exact arithmetic
• control/phaseplot.py — separatrices() uses the real part of the
  eigenvector, avoiding complex initial conditions

Matplotlib: warning no longer emitted

• control/tests/ctrlplot_test.py — drop the pytest.warns(UserWarning, match="Tight layout not applied") check in
  test_pole_zero_subplots(), which no longer fires

Platform-dependent numerics

• control/tests/nyquist_test.py — test_nyquist_indent_near_imaginary_axis()
  now constructs the system with ct.zpk(), placing the poles near the
  imaginary axis explicitly instead of relying on the roots of a
  polynomial, which gave inconsistent indent signs
• control/lti.py, doc/intro.rst — bandwidth() doctests use
  # doctest: +ELLIPSIS with ~14 significant digits. The value comes from
  a scipy.optimize.root_scalar bisection, so the last digit or two of
  the repr varies across SciPy/BLAS builds; the full-precision literals
  passed locally but failed on GitHub runners. +ELLIPSIS keeps the
  examples verified, unlike the +SKIP used elsewhere for
  platform-dependent output, and trim_doctest_flags (on by default)
  strips the flag comment from the rendered HTML.

Testing

make doctest in doc/: 728 tests, 0 failures
(Python 3.14.6, NumPy 2.5.1, SciPy 1.18.0).

AI disclosure

Per the AI policy referenced in README.rst: the bandwidth() doctest
change was written with assistance from Claude Code. I have reviewed it
and take responsibility for its content.