[2026/spec]: improve type hints

[LoicRiegel]

Inspired by this great YouTube comment and its replies.

What changed

• used from __future__ import annotations because Predicate type hint is used inside Predicate class
• Improved typing around rule definitions and factories using ParamSpec and Concatenate
• Changed the order of parameters in the rule functions. Function parameter order was standardized to subject first, then extra parameters because Concatenate only supports prepending fixed parameters before ParamSpec, not appending after it.

Hope it can help other viewers :)