[checkpoint] Restore GRL Sokoban episodes

Parent

Child of #3219 and #3024.

Problem and state owner

resources_servers/grl_sokoban/app.py owns a live Sokoban environment, board/player/box state, counters, observation, reward, done flag, and RNG state outside generic session storage.

Proposed adapter

Provide an explicit JSON codec and recreate a fresh environment from configuration plus serialized logical/RNG state.

Implement explicit checkpoint opt-in, route classification, export/restore/retire hooks, execution-to-session binding, revision reporting, atomic replacement under attempt N+1, and stale-attempt fencing.

Acceptance criteria

• The same post-restore action yields identical board, observation, reward, counters, and terminal status.
• Checkpoint after multiple mutations, replace the server process, restore, and compare the next operation with an uninterrupted execution.
• Corrupt or incomplete bulk state activates no sessions.
• Attempt N cannot mutate or retire restored attempt N+1.
• Terminal cleanup is idempotent and releases restored state exactly once.

Start with resources_servers/grl_sokoban/app.py and read parent issues #3219 and #3024 for the surrounding session and environment design. Trace the live board, counters, observation, reward, done flag, and RNG state, then map the proposed checkpoint hooks to the acceptance criteria. Done means deterministic restore, safe rejection of corrupt bulk state, stale-attempt fencing, and idempotent cleanup.