fix(blocks): one base point convention for creation, redefine, insert, explode, XCLIP and REFEDIT

world = insert_point + Rot * Scale * (P - base_point). Attribute placement, EXPLODE, XCLIP storage, REFEDIT's working copy, the UCS reframe of attributes and the dynamic-block visibility grip ignored the base point and were offset by Rot*Scale*base. They now share insert_transform_with_base. Creation stores a ZERO base and redefine resets it; a no-op redefine moves nothing; the Block Definition dialog prefills the base from the definition. XCLIP filters stored by older builds keep their old displacement.

src/scene/mod.rs

@@ -26,6 +26,8 @@ pub enum BlockScalePolicy {
 
 // Topic submodules split out of this root (each contributes `impl Scene`
 // blocks and/or free functions). Pure text-move from the original mod.rs.
+#[cfg(test)]
+mod block_base_point_tests;
 mod boundary;
 mod camera_ops;
 pub(crate) mod centerline;