fix(command_queue): deleteFile now cascade-deletes live artboards and their state machines

[mfazekas]

CommandServer::m_fileDependencies[file] is created empty at loadFile and walked by deleteFile to cascade-delete dependents, but instantiateArtboard never appends to it — so the cascade is always a no-op. Deleting a file while artboards instantiated from it are still alive orphans them (and their state machines) server-side, with no remaining documented cleanup path. Existing tests mask this by always deleting artboards explicitly before the file.

This PR:

1. Registers the artboard in m_fileDependencies at instantiateArtboard, mirroring how instantiateStateMachine registers in m_artboardDependencies.
2. Un-registers it in cleanupArtboard, so explicit create/delete cycles on a long-lived file don't grow the dependency vector unboundedly.
3. Makes deleteFile detach the dependency list before cascading (since cleanupArtboard now mutates it), and erases the file only after the cascade, since dependent artboards and state machines may reference file-owned data.

Tests added to command_queue_test.cpp (the first one fails without the fix):

1. deleteFile with two live artboards + state machines cleans all of them up.
2. Explicit deletes followed by deleteFile stay idempotent.
3. Commands on cascade-deleted handles go through the normal unknown-handle error path.

Full unit test suite passes.