Resolve the DIMLFAC sign convention and add the viewport measurement seams

DXF gives DIMLFAC a sign convention: a positive value scales every
dimension, a negative one applies |DIMLFAC| only to dimensions that live in
a paper-space layout (where it compensates a viewport's scale). The linear
formatter multiplied by the raw signed value, so a drawing with negative
DIMLFAC (e.g. 115448.dxf) rendered negative measurements.

Resolve the sign once, in `resolved_dimension_style`, the single place an
effective style is derived, so dimension text, alternate units, exploded and
baked text and the properties panel all agree. `extended_data` keeps the raw
signed value, so DXF/DWG export is unchanged. The properties "Measurement"
row now reports the value the text shows rather than the raw distance, while
`actual_measurement` (group 42) stays raw as the format requires.

Also adds the PR2 seams the dimension work builds on:
  * MeasurementScale::user_lfac_for_space / viewport_dimlfac_override —
    the persisted representation of viewport compensation.
  * Scene::viewport_frame and paper-point viewport resolution.
  * Model-geometry hit testing through a content viewport, including
    descent into block instances with the instance transform baked in.

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

src/scene/mod.rs

@@ -41,6 +41,7 @@ pub mod named_parameters;
 mod named_parameters_persist;
 mod mspace;
 pub mod viewport_ref;
+pub mod viewport_dim_seam;
 mod page_setup;
 mod paper;
 mod preview;