v1.9.0: phased checkpoints + resumable runs

A long run that crashes, hits a budget, or is stopped should not start over. Because every step is already witnessed durably, the ledger is the resume state.

• Resume: dispatch_plan(plan, ledger, executor, resume=True) (and Orchestrator.submit_plan(plan, resume=True)) reuses every task that already has a witnessed successful result and re-runs only the missing or failed ones. A resume entry records which tasks were reused. Resume reuses the verified record; it never regenerates it with a model.
• Phased checkpoints: checkpoint_each_wave=True witnesses a checkpoint entry (the Merkle root so far) and syncs the ledger after each wave, a re-checkable savepoint and the durability point for batched storage (v1.8).
• The summary and forum bench report checkpoints and resumes.

Pure standard library, deterministic. 223 tests, plus 2 gated real-model tests.

Install: pip install forum-engine