compound: optional step on a numeric binding

[defiantnerd]

A binding maps an attribute to a property linearly - scale * x + offset - which
is right for a continuous control and wrong for a segmented one. A meter drawn as
discrete LED segments, a knob that should land on detents, a stepped bar: all of
them need the display quantised while the attribute stays continuous, and that
was not expressible. The alternative was quantising at the source, which changes
the value everything else reads.

step on CompoundBinding snaps the attribute value to the nearest multiple
before the linear map, so one step is the same fraction of the range whatever the
property it drives is measured in. 0 (the default) is smooth, so every existing
binding is untouched.

No ABI change

There is no bind-with-step entry point in the vtable and this needs none: the step
travels through the existing float setter as "<prop>.step" (e.g. "width.step"),
the same route stroke_dasharray takes to travel as a string. It is also kept in a
per-layer bind_steps map, so setting it before the bind - or re-binding the prop
afterwards - does not lose it, and setting it on an unbound prop cannot conjure a
phantom binding that would read attribute "" as 0.

The loader parses "step" inside a bind object and the serializer emits it when
non-zero, so a component document round-trips.

Files

• hosts/shared/compound.h - the field, the quantisation in eval_binding_float,
  the "<prop>.step" setter, and apply_bind preserving it.
• hosts/shared/component_loader.h - parse + emit.
• include/neui/d/compound.h - the prop and the bind semantics documented.
• tests/ - the quantisation itself (rounding at the halfway point, reaching the
  full range, step 0 leaving the value alone), both setter orders plus the
  re-bind, and the document round-trip.

Testing

neui_tests: 317 cases / 1863 checks pass, 4 of them new.