fix(text): grip-drag no longer hides text (#316)

Dragging text by a grip hid it until release. The dragged entity is
hidden from the base wire set and shown as a preview overlay, but its
SDF glyph quads (text_verts) were never uploaded for the preview, so
the text vanished mid-drag.

Add a per-frame preview_text GPU buffer (sibling to preview_wires). A
rigid whole-entity move of pure text (TEXT/MTEXT, Square insertion grip)
slides the drag-start glyphs by the drag delta instead of re-shaping the
run every move; reshape grips (MTEXT width), dimensions and wire
entities re-tessellate and feed the same buffer.

Also fold preview_text into render_signature: a pure-text slide leaves
preview_wires empty, so without it the scene-render cache froze the
dragged text at its first frame (grip moved but text stayed, then
jumped to place on release).

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

src/scene/preview.rs

@@ -14,11 +14,18 @@ impl Scene {
         self.preview_wires = wires;
     }
 
+    pub fn set_preview_text(&mut self, verts: Vec<crate::scene::pipeline::text_gpu::TextVertex>) {
+        // Overlay glyphs — same reasoning as `set_preview_wires`: no geometry
+        // bump. Uploaded to a dedicated per-frame text buffer in `prepare`.
+        self.preview_text = verts;
+    }
+
     pub fn clear_preview_wire(&mut self) {
         // No geometry bump — see `set_preview_wires`. Dropping the overlay
         // flips the wire content id back to the base tessellation id, which
         // re-uploads the base wires (without the preview) on the next frame.
         self.preview_wires = vec![];
+        self.preview_text = vec![];
         self.interim_wire = None;
     }