chore: mark fill_tris_low unpaired sites with FIXME

dimension.rs (x2), multileader.rs (x3), table.rs (x1) emit fill_tris
without a corresponding fill_tris_low. These will trip the debug_assert_eq!
added in the preceding commit when the entity appears inside a block
definition. Each site is marked with a FIXME referencing block_cache.rs:1397
and the correct fix pattern (split_ds on each vertex).

src/entities/table.rs

@@ -755,6 +755,9 @@ pub fn tessellate_table(
                 plinegen: true,
                 vp_scissor: None,
                 fill_tris,
+                // FIXME: fill_tris_low left empty — needs double-single split to match
+                // fill_tris. Any geometry from this path inside a block definition will
+                // trip the debug_assert_eq! in emit_wire (block_cache.rs:1397).
                 fill_tris_low: Vec::new(),
             }
         };