Register missing broadcasts when importing a sprite

[Mistium]

Fixes #3.

When a sprite is imported (vm.addSprite), installTargets calls fixUpVariableReferences() to merge or create any global variables the sprite's blocks reference, but it fetched references without the optIncludeBroadcast flag, so BROADCAST_OPTION fields were skipped and the broadcast variable was never created on the stage. Broadcast blocks bail silently when lookupBroadcastMsg finds nothing. In the editor, scratch-blocks masks this by creating the variable during workspace load, which is why "see inside" made the project work.

The fix passes the broadcast flag so broadcast references flow through the same machinery as variables and lists: a broadcast whose name already exists on the stage gets its block fields re-pointed to the existing variable's id, and a new broadcast is created on the stage under the imported id. The _broadcastCleanupNeeded sweep only deletes broadcasts no block references, so the new ones survive.

Compared to the PenguinMod fix linked in the issue, this also handles the event_whenbroadcastreceived hat field, merges with existing same-named broadcasts instead of creating duplicates, and covers sprite2 imports.

Added a unit test covering both paths (merge with existing broadcast, create missing one); test/unit/engine_target.js passes 35/35.

🤖 Generated with Claude Code

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