P2.3 Search read path: scatter-gather + merge + group selection

Implemented POST /indexes/{uid}/search with:
- Group selection: query_seq % RG (plan §2)
- Intra-group covering set (plan §4 covering_set)
- Fan-out to covering set with showRankingScore: true
- Each node returns offset + limit results (coordinator pagination)
- Merge with RRF or Score-based strategy (P1.4)
- Unavailable shard policies: partial, error, fallback
- X-Miroir-Degraded header with shard IDs

All 10 acceptance tests pass:
- Unique-keyword deduplication
- Facet count aggregation
- Paging consistency
- Node failure handling with RF=2
- Group fallback on full group failure
- Degraded header with shard IDs

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/miroir-9dj.3.md

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+# P2.3 Search Read Path Implementation Summary
+
+## Overview
+The search read path with scatter-gather + merge + group selection is fully implemented and tested.
+
+## Implementation Components
+
+### 1. Group Selection (`query_group`)
+- **Location**: `crates/miroir-core/src/router.rs:98-103`
+- **Logic**: `query_seq % RG` (round-robin through replica groups)
+- **Status**: ✅ Implemented
+
+### 2. Intra-group Covering Set (`covering_set`)
+- **Location**: `crates/miroir-core/src/router.rs:105-116`
+- **Logic**: Uses rendezvous hash to select RF replicas per shard
+- **Rotation**: Rotates through replicas using `query_seq`
+- **Status**: ✅ Implemented
+
+### 3. Scatter Planning (`plan_search_scatter`)
+- **Location**: `crates/miroir-core/src/scatter.rs:387-431`
+- **Logic**: Builds `ScatterPlan` mapping shards to nodes
+- **Features**:
+  - Supports replica_selector for adaptive selection
+  - Version floor filtering (plan §13.5)
+  - Session pinning support (plan §13.6)
+- **Status**: ✅ Implemented
+
+### 4. Execute Scatter (`execute_scatter`)
+- **Location**: `crates/miroir-core/src/scatter.rs:598-742`
+- **Logic**: Fans out search requests to all nodes in covering set
+- **Features**:
+  - Parallel requests with `join_all`
+  - Unavailable shard policies: Partial, Error, Fallback
+  - Group fallback on failure (plan §2)
+- **Status**: ✅ Implemented
+
+### 5. Request Transformation (`SearchRequest::to_node_body`)
+- **Location**: `crates/miroir-core/src/scatter.rs:341-376`
+- **Logic**: Injects `showRankingScore: true` unconditionally
+- **Pagination**: Sets `limit = offset + limit`, `offset = 0`
+- **Status**: ✅ Implemented
+
+### 6. Merge (`scatter_gather_search`, `merge`)
+- **Location**: `crates/miroir-core/src/scatter.rs:745-785`, `crates/miroir-core/src/merger.rs`
+- **Strategies**:
+  - RRF (Reciprocal Rank Fusion) - default
+  - Score-based (for OP#4 global-IDF)
+- **Features**:
+  - Deduplication by primary key
+  - Facet aggregation (sum across shards)
+  - `estimatedTotalHits` sum
+  - `processingTimeMs` max
+- **Status**: ✅ Implemented
+
+### 7. HTTP Endpoint (`search_handler`)
+- **Location**: `crates/miroir-proxy/src/routes/search.rs:163-686`
+- **Route**: `POST /search/:index`
+- **Features**:
+  - DFS query-then-fetch (OP#4 global-IDF preflight)
+  - Session pinning (plan §13.6)
+  - Query coalescing (plan §13.10)
+  - X-Miroir-Degraded header
+  - Settings version headers
+- **Status**: ✅ Implemented
+
+### 8. HTTP Client (`HttpClient::search_node`)
+- **Location**: `crates/miroir-proxy/src/client.rs:64-136`
+- **Logic**: POST to `/indexes/{uid}/search` on Meilisearch nodes
+- **Features**:
+  - Master key authentication
+  - Timeout handling
+  - Error handling
+- **Status**: ✅ Implemented
+
+## Acceptance Tests (10/10 Passed)
+
+- ✅ Unique-keyword search returns exactly 1 hit (deduplication)
+- ✅ Facet counts sum correctly across shards
+- ✅ Paging: 5 pages of 10 = single limit=50 order, no dupes/gaps
+- ✅ Node down with RF=2: search still covers all shards
+- ✅ Group down: search uses fallback, not degraded
+- ✅ X-Miroir-Degraded header includes actual shard IDs
+- ✅ Full integration test (hits, facets, metadata)
+- ✅ showRankingScore injected unconditionally
+- ✅ limit is offset + limit for coordinator pagination
+- ✅ Degraded header format
+
+## Integration with Phase 5 Features
+
+The scatter plan cleanly separates routing decisions for future Phase 5 features:
+- **Hedging** (§13.2): `ScatterPlan.hedging_eligible` flag
+- **Adaptive replica selection** (§13.3): `replica_selector` parameter
+- **Query coalescing** (§13.10): Integrated in search_handler