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 <noreply@anthropic.com>

src/app/mod.rs

@@ -9,6 +9,7 @@ pub(crate) mod config;
 pub use automation::{export_headless, serve};
 mod command_driver;
 pub(crate) mod commands;
+mod dim_viewport;
 mod document;
 mod drafting_settings;
 pub(crate) mod expr_eval;
@@ -508,6 +509,12 @@ pub(super) struct OpenCADStudio {
     cursor_size: i32,
     /// Selection-box size setting (PICKBOX, 0..=50).
     pick_box: i32,
+    /// PR2: accepted snaps for the points the active dimension command has
+    /// collected, in collection order. Transient command state — the committed
+    /// result is an ordinary dimension entity, so undo/redo needs nothing here.
+    pub(crate) dim_accepted_snaps: Vec<crate::scene::viewport_ref::AcceptedSnap>,
+    /// Name of the command `dim_accepted_snaps` belongs to.
+    pub(crate) dim_accepted_snaps_owner: Option<String>,
     /// Use REFEDIT rather than BEDIT when double-clicking an attribute-free block.
     double_click_block_refedit: bool,
     /// Open ATTEDIT when double-clicking a block with attributes.
@@ -3634,6 +3641,8 @@ impl OpenCADStudio {
             zoom_factor: 60,
             cursor_size: 5,
             pick_box: 3,
+            dim_accepted_snaps: Vec::new(),
+            dim_accepted_snaps_owner: None,
             double_click_block_refedit: false,
             double_click_block_attedit: true,
             grip_object_limit: settings::DEFAULT_GRIP_OBJECT_LIMIT,