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/mod.rs

@@ -752,6 +752,10 @@ pub struct Scene {
     pub selection_filter: HashSet<String>,
     /// In-progress preview wires while a command is active (rubber-band + object ghosts).
     pub preview_wires: Vec<WireModel>,
+    /// In-progress preview SDF glyph quads (grip drag / command preview). Rides
+    /// a per-frame GPU buffer separate from the epoch-cached base text so the
+    /// dragged text stays visible while it's hidden from the base set (#316).
+    pub preview_text: Vec<crate::scene::pipeline::text_gpu::TextVertex>,
     /// Committed-segment wire drawn during multi-point commands (normal colour).
     pub interim_wire: Option<WireModel>,
     pub camera_generation: u64,
@@ -995,6 +999,7 @@ impl Scene {
             transparency_display: true,
             selection_filter: HashSet::default(),
             preview_wires: vec![],
+            preview_text: vec![],
             interim_wire: None,
             camera_generation: 0,
             geometry_epoch: GEOMETRY_EPOCH.fetch_add(1, Ordering::Relaxed),