Move projection maths onto the projection variants - #86

Follow-up to #85. That PR introduced the Orthographic | Perspective sum type
but left the projection maths as match/isinstance dispatch spread across
ViewState and three renderers. This moves that maths onto the variants behind
a Projection abstract base class, so adding a third variant is a one-class
change rather than an edit to every dispatch site.

Why

Projection was data only, so every question about a projection -- how a
camera point maps to the screen, a sphere's silhouette radius, the worst-case
magnification, the bond-cap eye distance, whether a point reaches the eye plane
-- was answered by a match outside the type. There were four such dispatch
sites plus assert_never guards. Those are all projection concepts, so they
belong on the projection.

Projection becomes an ABC (the existing LegendItem pattern) with five
abstract methods; Orthographic and Perspective implement them. The dispatch
sites collapse to method calls, the assert_never guards are replaced by
definition-time enforcement (an incomplete variant cannot be instantiated), and
the types move into a new hofmann/model/projection.py -- returning
view_state.py to being about ViewState (434 to 254 lines).

Behaviour

Rendered output is unchanged, bit-for-bit, verified with a vertex-level harness
against main after every commit. Two internal cleanups ride along, both
behaviour-neutral: project_camera now returns xy only (the scale it
returned was discarded by every caller), and the duplicated
_foreshortening_distance helper collapses to an eye_distance property.

Notes for review

• The two assert isinstance(proj, Perspective) sites sit inside
  if proj.reaches_eye_plane(...), which only Perspective returns True
  from -- so they are unreachable for a parallel variant and narrow the type
  for the warning's f-string. They are not dispatch a third variant would edit.
• Projection.__abstractmethods__ is pinned by a test, so a method silently
  losing @abstractmethod fails rather than letting an incomplete variant
  through.
• One interface method does not yet generalise beyond the two current variants:
  eye_distance is a scalar z-axis distance, which a genuinely oblique
  projection could not express. Left as-is deliberately rather than reshaped for
  an unwritten variant; the follow-up oblique work will generalise it.