Add to_varstore() compatibility shims on DuelingQNetwork and
DistributionalDuelingQNetwork so test/example code can rebuild a
GpuVarStore snapshot when needed. Delete dead tests that referenced
removed DQNAgent, PrioritizedReplayBuffer, and ReplayBufferType.
Fix action index references (action_19/21 -> action_28/30) and
type annotation issues (sin ambiguity, remainder operator).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>