refactor(canvas): review fixes for the busy flag, the comparison note and the bundle size check

- busyReason had no reader: the chat engine sets isBusy through the store's
  generic set, like every other flag.
- The isSameDocument comment says the two "same document" rules disagree
  in both directions, so neither is a subset of the other.
- The import boundary test bundles minified with one pako (the MCP core
  resolves its own copy) and caps the canvas core at 160 KB (134 KB now).

lib/diagram-diff.ts

@@ -288,7 +288,10 @@ export function sameFileVars(a: string | null, b: string | null): boolean {
  * page settings and file variables. This is the web app's one rule for "is
  * this version on the canvas" (versions, compare, one-step commits). The
  * MCP server's edit gate has its own (contentFingerprint): it compares the
- * cells as written, and leaves page settings and file variables out.
+ * cells as written, and leaves page settings and file variables out. The
+ * two disagree both ways (a background change is a difference only here;
+ * a geometry written "40.0" and "40" is one only there), so neither is a
+ * subset of the other.
  */
 export function isSameDocument(a: string, b: string): boolean {
     const pagesOf = (doc: Document) => {