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/cache/block_cache.rs

@@ -419,7 +419,7 @@ fn tessellate_sub_local(
     // before casting to f32 — same precision-preservation trick used for
     // top-level entities, applied per-defn.
     let wires_out = tessellate::tessellate(
-        doc, h, sub, false, sub_color, pat_len, pat, lw_px, anno_scale, None, bg_color,
+        doc, h, sub, false, sub_color, pat_len, pat, lw_px, anno_scale, None, bg_color, false,
     );
     if wires_out.is_empty() {
         return vec![];