Resolve dimension references through viewport and block chains

An `AssocDimensionReference` addresses a feature through a chain of
handles — dimension -> [viewport] -> [insert ...] -> entity -> feature —
stored in `xrefs` (AutoCAD's `mainObjectIds`). The existing resolver read
only `xrefs[0]`. That is right for a model-space dimension on top-level
geometry and wrong for everything else: a paper-space dimension measuring
through a layout viewport has the VIEWPORT first, so resolution stopped
on a viewport entity, found no curve and gave up. Nine of the ten
association objects in the 115448 regression drawing are that shape.

`dimension_assoc_chain` walks the chain positionally, which is all the
file gives us: a leading VIEWPORT is the trans-space hop, INSERTs after
it are the block-instance path (with their composed transform), and the
first handle that is neither is the geometry. A chain of nothing but
INSERTs ends on the innermost one, which is how a block reference's own
insertion point is dimensioned. A handle that is no longer in the
document comes back as `ChainError::Missing` rather than as a silent
failure, so the caller can tell a broken link from an unsupported one.

The module also evaluates the osnap feature itself, in the entity's own
coordinates: centre, midpoint, quadrant, node/insertion, nearest,
perpendicular, tangent (closed form on a circle or arc; a Newton
refinement of (C(u) - P) . C'(u) = 0 on a spline, seeded from the stored
parameter — the shape of the 115448 tangent-to-spline reference), and
intersection / apparent intersection against the second object chain.
Types we do not evaluate return None and keep their stored data, so an
unsupported reference is left unresolved instead of being replaced by a
guess.

`Transform` has no inverse of its own and a block path is always affine,
so the 3x3 cofactor inverse lives here too. All arithmetic is f64.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/scene/mod.rs

@@ -30,6 +30,7 @@ mod boundary;
 mod camera_ops;
 pub(crate) mod centerline;
 pub(crate) mod dimension_assoc;
+pub(crate) mod dimension_assoc_chain;
 pub(crate) mod centermark;
 mod dwg_native_constraints;
 mod entity;