feat(text): SDF for synthesized-table cell text + guard table pick box

An in-app-created table (no baked *T### block) synthesised its cell text via
layout_mtext as stroke polylines — the last text path still rendering strokes
while SDF is the default (imported/baked tables already render SDF via the
recursive Text arm).

- tessellate_table: when SDF is on, build each cell's glyph quads from the
  run-carrying layout and collect them into one text-carrying wire (per-vertex
  cell colour, glyph-bounds AABB); the stroke/fill accumulation is the SDF-off
  fallback. Mirrors the MLEADER SDF branch.
- tess.rs synthesized-table path: guard the whole-table AABB clobber on
  (!points.is_empty() || !fill_tris.is_empty()) — matching the dim / mleader /
  baked-block paths — so the empty SDF-text wire keeps its tight glyph-box AABB
  and the pick box hugs the text instead of the whole table. Found by the
  adversarial review pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/convert/tess.rs

@@ -437,7 +437,12 @@ pub(crate) fn tessellate_entity(
             let aabb = entity_aabb(e);
             for w in &mut wires {
                 w.aci = aci;
-                w.aabb = aabb;
+                // Empty SDF-text cells keep their tight glyph-box AABB; only
+                // stroke/fill wires take the whole-table box as a broad-phase
+                // pick hint (matches the dim / mleader / baked-block paths).
+                if !w.points.is_empty() || !w.fill_tris.is_empty() {
+                    w.aabb = aabb;
+                }
             }
             return wires;
         }