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/pipeline/text_gpu.rs

@@ -120,6 +120,29 @@ pub fn push_glyph_vertices(
     }
 }
 
+/// Slide every glyph vertex by a world-space delta, re-splitting the
+/// double-single position. Lets a grip drag move already-shaped text by
+/// translating the drag-start glyphs each frame instead of re-tessellating
+/// (re-shaping) the run on every cursor move (issue #316).
+pub fn translate_verts(verts: &[TextVertex], delta: [f64; 3]) -> Vec<TextVertex> {
+    verts
+        .iter()
+        .map(|v| {
+            let wx = v.pos[0] as f64 + v.pos_low[0] as f64 + delta[0];
+            let wy = v.pos[1] as f64 + v.pos_low[1] as f64 + delta[1];
+            let wz = v.pos[2] as f64 + v.pos_low[2] as f64 + delta[2];
+            let (xh, xl) = split_ds(wx);
+            let (yh, yl) = split_ds(wy);
+            let (zh, zl) = split_ds(wz);
+            TextVertex {
+                pos: [xh, yh, zh],
+                pos_low: [xl, yl, zl],
+                ..*v
+            }
+        })
+        .collect()
+}
+
 // ── Atlas texture ───────────────────────────────────────────────────────────
 
 /// The shared glyph atlas on the GPU: a single-channel (R8) SDF texture plus