fix(leader): suppress oversized arrowheads, keep them precise at UTM

Three defects made a leader's arrowhead render as one stray line far
longer than the leader itself, at UTM coordinates:

- The arrowhead was drawn even when it is larger than the first leader
  segment it sits on. A DIMASZ sized for a bigger plot (here 6 units on a
  ~1-4 unit pointer) painted a stroke longer than the whole leader.
  AutoCAD omits the arrowhead when it does not fit the first segment, so
  gate the arrow on `arrow_size <= first_segment_len`.

- Leader geometry was built as absolute f32. At UTM coordinates (Y ~ 4.5M,
  f32 ulp ~ 0.5) the oblique arrow tick's small cross-slope quantised
  away, collapsing the 45-degree tick into a flat horizontal line. Build
  the path and arrow in absolute f64 — the arrow around a local origin so
  append_arrow's f32 math stays precise — then RTE-split into the double
  single (points_low / fill_tris_low) at the end.

- The `_Oblique` / `_Archtick` arrowhead block mapped to ArrowKind::Tick
  with the full DIMASZ, but the tick emitter draws `size` to either side
  (half-length semantics, matching DIMTSZ). Pass half so an oblique
  arrowhead sizes to DIMASZ like every other arrowhead, not double.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/convert/tessellate.rs

@@ -1551,7 +1551,12 @@ fn arrow_from_block_name(name: Option<&str>, dimasz: f32) -> ArrowKind {
         "ORIGIN" | "ORIGIN2" | "ORIGININDICATOR" | "ORIGININDICATOR2" => {
             ArrowKind::Origin { size: dimasz }
         }
-        "OBLIQUE" | "ARCHTICK" => ArrowKind::Tick { size: dimasz },
+        // `ArrowKind::Tick` draws the stroke `size` to either side of the tip
+        // (total 2·size — its `size` is a half-length, matching DIMTSZ). As an
+        // arrowhead block, though, DIMASZ is the stroke's *full* length like
+        // every other arrowhead here, so pass half of it — otherwise the
+        // oblique tick renders twice the intended DIMASZ.
+        "OBLIQUE" | "ARCHTICK" => ArrowKind::Tick { size: dimasz * 0.5 },
         "BOXFILLED" => ArrowKind::Box_ {
             size: dimasz,
             filled: true,