fix(mtext): show preview glyphs and make Enter break the line

Two defects in the in-app MTEXT editor, both surfaced once the preview
text became visible:

1. Invisible preview text. After the SDF text migration, tessellate emits
   glyphs as SDF quads on `text_verts` and leaves the polyline `points`
   empty, but the editor preview draws on a 2D canvas that can't run the
   SDF shader — so only the selection box showed. Add a `force_text_strokes`
   parameter to tessellate: when set, TEXT/MTEXT run-groups also emit their
   glyph outline strokes as points. The preview passes true (real stroke
   font, aligned with glyph_boxes); every other caller passes false and
   keeps SDF-only text.

2. Enter did not start a new line. mtext_type inserted a literal newline,
   but the MTEXT layout only breaks on the `\P` code (a raw 0x0A is a break
   only to the caret logic). Normalise every literal newline — from Enter or
   a multi-line paste — to `\P`, so the line breaks and the saved value
   stays standard.

Closes #308

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

src/entities/insert.rs

@@ -240,6 +240,7 @@ pub(crate) fn append_insert_attribute_wires(
             anno_scale,
             None,
             bg_color,
+            false,
         );
         // Use the INSERT's handle so selection / picking groups attribute
         // text with the parent insert instead of treating it as a stray text.

src/entities/multileader.rs

@@ -1357,6 +1357,7 @@ impl MultiLeaderTess for MultiLeader {
                         1.0,
                         None,
                         bg_color,
+                        false,
                     );
                     for w in &mut sub_wires {
                         w.name = name.clone();