Port 6 audit-found bugs from the upstream OSNAP fix branch

Ports the fixes from fix/osnap-circle-arc-ellipse-intersection (and
this session's audit follow-up) onto this branch. All are independent
correctness/reliability fixes, unrelated to the parametric-constraint-
system work also in progress here.

1. OSNAP Intersection snap used a tessellated approximation instead of
   exact geometry (GitHub discussion #1052): a circle/arc always
   tessellates at a fixed 48 segments in the resident wire set
   regardless of radius or zoom, so every circle/arc intersection snap
   was off by construction. Added exact_circle_intersections, a
   closed-form, frame-independent circle/arc-vs-circle/arc solver,
   wired into both Intersection-snap code paths ahead of the
   tessellated sweep. This branch's TangentGeom predates the
   PlanarEllipse variant added upstream, so the ellipse/line
   generalization from the upstream fix isn't ported here — this
   covers the literal reported bug (a circle-circle case) using only
   variants this branch already has.

2. Unbounded MINSERT allocation: array_offsets() reserved one Vec
   element per array instance with row/column counts parsed as
   unchecked u16s — a corrupt 65535x65535 pair requests ~96 GiB.
   Fixed with a documented instance budget (20,000), enforced at both
   the load-time corrupt-entity filter and a render-graph failsafe.

3. MINSERT spacing was scaled by the block's own scale:
   insert_instance_transform composed the offset translation before
   the instance transform, so x/y block scale multiplied the row/
   column spacing too. Fixed by orienting the offset with rotation +
   OCS only, then translating the already-scaled instance transform.

4. Automation "open" skipped the finalization (block-origin
   normalization, raster-path resolution, source_path) and corrupt-
   entity purge a UI open always runs. Added
   io::load_bytes_finalized(path, bytes), used by automation's "open"
   handler, which now also reports the purge count.

5. Material maps were uploaded without checking
   device.limits().max_texture_dimension_2d, unlike raster-image
   rendering (which already tiles to that limit). A mesh material is
   sampled by UV, so it can't use the same tiling trick — instead,
   upload_rgba_texture now downscales an oversized image before
   upload, falling back to the 1x1 material color if malformed.

6. Sketch-constraint persistence/undo, audit-found while reviewing
   this branch's own constraint system:
   - Clearing every constraint in a scope after a prior non-empty save
     left the stale XRecord in place, resurrecting the deleted
     constraints on the next load. materialize_sketch_constraints_for_
     save now actively removes the XRecord when a scope goes back to
     empty.
   - ERASE removing an entity took its constraints with it (design
     doc's deletion policy), but sketch_constraints lives on Scene,
     outside document/document.objects, so nothing captured that
     mutation for undo — the erased geometry came back on undo, but
     its constraints didn't. refresh_sketch_constraints's deletion
     pass now records the touched scope's before-image into the same
     UndoRecording the entity removal rides in, threaded through a new
     sketch_constraints channel on DeltaSnapshot, so one undo restores
     both together. HistorySnapshot::Delta is now boxed (clippy
     flagged the resulting size skew across undo/redo stack variants).

31 new/adapted tests across all six fixes. Full workspace suite passes
except the one pre-existing, unrelated font-test failure also present
on main.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/scene/sketch_constraints.rs

@@ -85,7 +85,7 @@ pub type ConstraintId = u32;
 /// One persisted constraint record: a friendly [`ConstraintKind`], the
 /// entities/points it relates, and — for a dimensional kind — the value
 /// driving it.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct SketchConstraint {
     pub id: ConstraintId,
     pub kind: ConstraintKind,