Record viewport associations for new dimensions

Removes PR3's `viewport_frame` seam in `viewport_ref.rs`; the frame is
PR1's camera-derived one everywhere.

The five dimension commands (DIMLINEAR / DIMALIGNED / DIMRADIUS /
DIMDIAMETER / DIMANGULAR) now record a real association when they
measure through a viewport. The commit path's one association gate
(`dimension_association_allowed`, added when PR2 was stacked) gains an
else-branch: where PR2 committed the dimension unassociated and PR1's
inference guard returned an empty source list, `Scene::attach_viewport_
dimension_association` now stores the whole chain — dimension ->
viewport -> block path -> entity -> feature — from the AcceptedSnaps PR1
captured, in definition-point slot order.

* `Scene::dimension_association_slot_points` names that order, including
  the radial dimensions `dimension_reference_points` does not cover.
* `OpenCADStudio::viewport_dimension_snaps` builds the slot array. A snap
  whose paper point is no longer the definition point it fills has its
  model point recomputed from that definition point through the frame:
  the object pick records one click for both extension origins, and a
  rotated DIMLINEAR projects its origins onto the measuring axis.
* Radial dimensions take their own path inside
  `attach_viewport_dimension_association`. Their single reference is an
  osnap type 10 carrying the radial marker (which circle / which polyline
  arc segment) and the angle around it; `source_reference` would hand
  back the generic curve-parameter marker -2, which
  `radial_source_for_marker` cannot find.

One measurement rule, documented in `viewport_ref::measurement_rule`:
`actual_measurement` is always the RAW measurement in the dimension's own
space (paper, for a viewport dimension), and the viewport compensation
lives only in the magnitude of the negative DIMLFAC override, applied
once by the formatter. PR2's persistence already followed it; PR3's
refresh did not — it substituted the model distance times
`model_factor()`, which made a refreshed dimension read `user_lfac` times
too much versus an identical freshly-created one. Refresh now writes the
paper measurement (a debug assert pins resolved paper points to the model
points through the frame), and `axis_distance` goes with it.

Block sub-entity snaps: verified that `block_cache` stamps the top-level
INSERT's handle onto every expanded and clipped sub-wire, so
`wire_source` can only ever report the INSERT — nested inserts included.
`OpenCADStudio::resolve_source_block_path` now descends from that INSERT
at accept time (using the object-pick path's existing
`resolve_measurable_entity`, so arbitrary depth, not just one level) and
fills `SnapSourceRef.block_path` with the real inner entity. Where the
descent cannot identify one — a non-planar-pickable type such as text,
hatch, spline or solid, or nesting past eight levels — the source stays
the INSERT with an empty path, which is coarser but never wrong. Both the
limitation and where it bites are documented on
`Scene::resolve_block_snap_source` and `SnapResult::source`.

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

src/app/update/viewport.rs

@@ -4173,6 +4173,7 @@ impl OpenCADStudio {
                 // `world_pt` exactly as before.
                 let pending = self.pending_click_snap.take();
                 self.record_accepted_snap(
+                    i,
                     pending.map(|(hit, _)| hit),
                     pending.and_then(|(_, frame)| frame),
                     world_pt,

src/app/update/viewport_snap.rs

@@ -102,11 +102,12 @@ impl OpenCADStudio {
     /// after ortho / polar / axis-lock / dynamic-input resolution.
     pub(crate) fn record_accepted_snap(
         &mut self,
+        tab: usize,
         snap: Option<SnapResult>,
         frame: Option<ViewportFrame>,
         committed: glam::DVec3,
     ) {
-        let accepted = match snap {
+        let mut accepted = match snap {
             // A viewport snap keeps its frame only when the hit really came
             // through it, so a paper-sheet snap never claims a model point.
             Some(hit) => {
@@ -115,9 +116,42 @@ impl OpenCADStudio {
             }
             None => AcceptedSnap::free(committed),
         };
+        self.resolve_source_block_path(tab, &mut accepted);
         self.push_accepted_snap(accepted);
     }
 
+    /// Replace a snap source that is a block INSTANCE with the entity inside
+    /// it that actually owns the snapped feature, plus the INSERT path.
+    ///
+    /// The snap engine can only report what the wire carries, and the
+    /// renderer stamps every wire of an expanded block with the *top-level
+    /// INSERT's* handle — nested inserts included. `SnapSourceRef` promises
+    /// the innermost entity in `source.handle` and the instance chain in
+    /// `block_path`, so the descent happens here, once, at accept time (not
+    /// per pointer move).
+    ///
+    /// When the descent cannot identify the inner entity the source stays the
+    /// INSERT with an empty path. See
+    /// [`crate::scene::Scene::resolve_block_snap_source`] for exactly when
+    /// that happens; the resulting association is coarser (it tracks the whole
+    /// block instance) but never wrong.
+    fn resolve_source_block_path(&self, tab: usize, accepted: &mut AcceptedSnap) {
+        let Some(source) = accepted.source.as_mut() else {
+            return;
+        };
+        if !source.block_path.is_empty() {
+            return;
+        }
+        if let Some((entity, path)) = self.tabs[tab]
+            .scene
+            .resolve_block_snap_source(source.source.handle, accepted.model_point)
+        {
+            source.source =
+                crate::command::DimensionAssociationSource::inferred(entity);
+            source.block_path = path;
+        }
+    }
+
     /// Append an already-built accepted snap, keeping the retention cap.
     ///
     /// Used by paths that are not point steps (the dimension object pick