fix: address review findings on merged #293/#292 hatch+snap

Three defects found reviewing the just-merged PRs:

- PLOTWINDOW forced corner snap (#293) only ran in the on_tick preview
  path, so the marker highlighted an endpoint but the click committed a
  plain snap — with the global OSNAP master (or Endpoint mode) off the
  corner landed at the raw cursor, defeating the feature. Apply the same
  snap_forced_corners branch at the click-commit recompute.

- build_dxf_pattern (#292) wrote the pattern line's LOCAL step into the
  world-frame HatchPatternLine.offset. The new prebaked reader
  inverse-rotates offset assuming world frame, so app-created hatches
  (HATCH command) collapsed their spacing by cos(angle) — ANSI31 at 45deg
  rendered 2.245 instead of 3.175 on both viewport and PDF/plot export.
  Rotate the local step into world here so it round-trips and the stored
  offset is format-correct for other CAD apps.

- far_from_origin_pattern_hatch_still_fills placed its offset ALONG the
  45deg lines (projects to k~0), so it never exercised the span-cap fix and
  passed even with the old absolute-index clamp restored. Move the offset
  perpendicular to the lines so |k| >> the cap. Add
  app_created_hatch_roundtrips_catalog_spacing to guard the offset-frame
  fix (verified: fails at 2.245 without it, passes at 3.175 with it).

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

src/scene/model/hatch_patterns.rs

@@ -45,10 +45,20 @@ pub fn build_dxf_pattern(entry: &PatternEntry) -> DxfPattern {
     pat.description = entry.description.clone();
     for ln in &entry.pat_lines {
         let angle_rad = (ln.angle_deg as f64).to_radians();
+        // The catalog stores the step (dx = shift along the line, dy =
+        // perpendicular spacing) in the pattern LINE-LOCAL frame. The DWG
+        // `HatchPatternLine.offset` is a WORLD-space vector — that is how real
+        // files store it and how `family_from_stored_line` reads it back (it
+        // inverse-rotates by the line angle). Emitting the raw local step here
+        // made the reader recover a rotated, too-dense spacing (e.g. ANSI31 at
+        // 45° collapsed 3.175 → 2.245). Rotate local → world by the line angle
+        // so the offset is format-correct and round-trips to the exact spacing.
+        let (ca, sa) = (angle_rad.cos(), angle_rad.sin());
+        let (ldx, ldy) = (ln.dx as f64, ln.dy as f64);
         pat.lines.push(HatchPatternLine {
             angle: angle_rad,
             base_point: Vector2::new(ln.x0 as f64, ln.y0 as f64),
-            offset: Vector2::new(ln.dx as f64, ln.dy as f64),
+            offset: Vector2::new(ldx * ca - ldy * sa, ldx * sa + ldy * ca),
             dash_lengths: ln.dashes.iter().map(|&d| d as f64).collect(),
         });
     }