fix(blocks): one block definition service for delete, rename and purge on every path

new src/scene/block_service.rs (references, delete, rename, purge candidates, name validation, one error type and message). The command-line RENAME, palette Remove, plugin API, control/MCP block_delete and PURGE all use it. Layout, xref, dynamic-block, anonymous and open-in-editor records are protected; PURGE removes an anonymous block only when nothing references it. PROTOCOL CHANGE: control/MCP block_delete refuses a referenced block unless force=true (erases referencing INSERTs in one undo step). New REST route DELETE /api/v1/blocks/{name}?force=true. MCP tool digest re-pinned. Undo labels are Delete block X, Rename block X to Y and Purge.

src/scene/mod.rs

@@ -6,6 +6,7 @@
 //   view    — camera, transforms, viewport, render pipeline driver
 //   cache   — block-definition and property caches
 pub mod annotative;
+pub mod block_service;
 pub mod cache;
 pub mod convert;
 pub mod creation_style;