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/scene/convert/tess.rs

@@ -204,6 +204,7 @@ pub(crate) fn tessellate_entity(
             1.0,
             world_per_pixel,
             bg_color,
+            false,
         );
         let ab = entity_aabb(e);
         for w in &mut wires {
@@ -594,6 +595,7 @@ pub(crate) fn tessellate_entity(
                     anno_scale,
                     world_per_pixel,
                     bg_color,
+                    false,
                 );
                 for w in &mut wires {
                     w.name = h.value().to_string();
@@ -654,6 +656,7 @@ pub(crate) fn tessellate_entity(
         anno_scale,
         world_per_pixel,
         bg_color,
+        false,
     );
     for b in &mut bases {
         b.aci = aci;