nodeconf: configuration epochs, change observers, config history store

[cheggaaa]

Groundwork for tree-node resharding (design: any-sync-node docs/resharding-plan.md).

Changes

• Configuration.Epoch: monotonically increasing version assigned by the coordinator on every publish (0 = pre-epoch configs); exposed via a new epoch field in NetworkConfigurationResponse (+ regenerated coordinatorproto).
• nodeconf.Service.ObserveChanges(observer): observers are called with the previous and new NodeConf whenever the active configuration is replaced (not for the initial one). The node's resharder uses this to trigger drain cycles immediately instead of waiting for a periodic timer.
• nodeconf.HistoryStore (optional Store extension) + implementation in nodeconfstore: retains the last 100 epochs per network on disk. Used by nodes to recompute earlier rings after a restart mid-migration and to authenticate draining nodes that were already removed from the current configuration.

Companion PRs

• nodeconfsource: configuration epochs + partition-overlap guardrail any-sync-coordinator#132 (epoch assignment + partition-overlap guardrail)
• Resharding: S3-mediated space migration between tree nodes any-sync-node#227 (the resharding machinery)

Testing

• new unit tests: observer dispatch (fires exactly once per change, not for the initial config), history store save/get/epochs/pruning
• full repo test suite passes

🤖 Generated with Claude Code

https://claude.ai/code/session_01KDgytoFFikB7tY4oRreWom

[github-actions]

New Coverage	58.7% of statements
Patch Coverage	71.6% of changed statements (48/67)

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