feat(checkpoint): union sharded generation cuts

[macandro96]

Summary

• export digest-bound generation-cut index fragments from each policy-model proxy
• accept the union of peer cut indexes in the authoritative model-ledger commit
• validate that sharded prefix checkpoints cover every prepared proxy

Stack

• depends on perf(checkpoint): scale recovery control paths #3755
• RL counterpart: feat(checkpoint): support sharded generation-prefix recovery RL#4323

Validation

• git diff --check passed
• the two-shard prefix-recovery functional test remains to run with the RL counterpart

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