deps: declare fbm as a runtime dependency, not a dev extra

[geekbeast]

Follow-up to #5 on the same branch — one last-minute packaging fix found while landing that work.

The problem

fbm was declared only in the dev extra and requirements-dev.txt, but it is imported at package load time:

• powersig/util/__init__.py:1 — from .fbm_utils import fractional_brownian_motion
• powersig/util/fbm_utils.py:3 — from fbm import FBM

Because __init__.py pulls fbm_utils in eagerly, importing anything under powersig.util executes that from fbm import FBM. That takes down the backends on a plain (non-dev) install:

• powersig/torch/algorithm.py:6 — from powersig.util.grid import get_diagonal_range
• powersig/cupy_backend/algorithm.py:5 — from powersig.util.grid import get_diagonal_range
• powersig/jax/utils.py:5 — import powersig.util.fbm_utils

All of them raise ModuleNotFoundError: No module named 'fbm' — i.e. the torch and cupy backends are unimportable from a released wheel.

The fix

Move fbm>=0.3.0 from the dev extra into [project].dependencies, and add it to requirements.txt. Removed from the dev extra and requirements-dev.txt, where it is now redundant.

fbm is safe to require unconditionally: 0.3.0 is a pure-Python py3-none-any wheel, four source modules, no compiled extensions, and numpy is its only dependency — which this project already requires. Nothing CUDA-specific or platform-specific, so it adds no constraint to the CUDA 13 stack landed in #5.

Verification

Per the commit message: the three imports above succeed, and tests/test_core_jax.py stays 21/21.

The import chain above was also confirmed statically against the branch tree.

CI note: the workflow installs .[jax-cpu,dev]. fbm leaving the dev extra doesn't remove it from that install — it moves into the core dependencies, which pip install . always resolves. So CI keeps getting fbm, and this change is exactly what makes a plain pip install powersig get it too.