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/app/update/command.rs

@@ -493,6 +493,8 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                     // once, and drop the preview.
                     if let Some(h) = self.grip_preview_handle.take() {
                         let i = self.active_tab;
+                        self.grip_text_verts = Vec::new();
+                        self.grip_text_slide = false;
                         if let Some(orig) = self.grip_original.take() {
                             if let Some(e) = self.tabs[i].scene.document.get_entity_mut(h) {
                                 *e = orig;