- Add NodeClient trait for HTTP calls to Meilisearch nodes (seam between pure miroir-core and networked miroir-proxy)
- Add ScatterPlan struct containing chosen_group, target_shards, shard_to_node mapping, deadline_ms, hedging_eligible
- Implement plan_search_scatter() pure function that constructs the covering set without I/O
- Implement execute_scatter() async function that fans out to nodes with partial-failure handling
- Add MockNodeClient for testing with pre-programmed responses/errors
- Add unit tests for plan construction, query group rotation, shard-to-node mapping, hedging eligibility, and scatter execution
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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