adds threshold momentum modules, which switch effects
once a machine's momentum passes halfway.

A threshold module behaves as one module while momentum is building, then flips to a different one
above 5 momentum (out of 10) — for example a productivity module that becomes a speed module once the
machine has been running a while. It flips back as soon as momentum falls to 5 or below. The mod generates a module for every ordered pair of entries in the same module family,
so both directions of a pair exist (Prod→Speed and Speed→Prod), across tiers 1-3. Their icons are
split: the left half shows what the module starts as, the right half what it turns into.

Pairs that would mix families (e.g. a clean module with a vanilla one) are skipped, which keeps the
number of combinations manageable.