Measure paper-space dimensions through the viewport they were picked in

DIMLINEAR / DIMALIGNED / DIMRADIUS / DIMDIAMETER / DIMANGULAR created on a
sheet now report the model measurement of the geometry seen through a
viewport, at any viewport scale, while definition points, the dimension line
and the text stay in paper coordinates and the annotation keeps its sheet
size (DIMSCALE is untouched).

A viewport frame is a similarity - uniform scale plus in-plane twist - so
projecting the model span onto the paper axis and compensating gives exactly
the raw paper measurement times 1/scale. No geometry is rewritten: the
compensation is persisted once as a negative DIMLFAC override on the
dimension and applied once by the linear formatter, so preview text,
dimension text, the properties measurement row and DXF/DWG export all agree.
User measurement multipliers fold into the same magnitude, and an explicit
text override still bypasses formatting entirely, so it is never re-scaled.
Angular dimensions are never compensated - a similarity preserves angles.

Mixed input (one point through viewport A, another through viewport B or on
the sheet) is refused rather than silently mis-measured: the command line
explains why and the dimension falls back to plain paper measurement.

A dimension measured through a viewport is left unassociated: its definition
points are projected paper coordinates, so inferring a paper-space
association would bind it to geometry that is not there. PR3 records the real
dimension -> viewport -> entity chain.

The result is one ordinary dimension entity carrying one style override, so
undo/redo works through the existing history with no new out-of-document
state.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/command.rs

@@ -2163,6 +2163,14 @@ pub trait CadCommand: Send {
     }
 
     /// Returns `true` when the command needs entity picking (hit-test) instead of point picking.
+    /// PR2: this command creates a dimension whose measurement must be
+    /// compensated when its points are picked through a paper-space viewport,
+    /// and whose explicit object pick may land on model geometry displayed
+    /// inside one. See `crate::app::dim_viewport`.
+    fn measures_through_viewports(&self) -> bool {
+        false
+    }
+
     fn needs_entity_pick(&self) -> bool {
         false
     }