P5.8.a: Verify anti-entropy fingerprint step implementation

Verified that the fingerprint step (plan §13.8 step 1) is fully implemented:
- Per-replica xxh3 digest over (pk || content_hash)
- Paginated iteration via filter=_miroir_shard={id}
- Streaming xxh3 digest folding seeded by shard_id
- Self-throttling with 10ms sleep between batches
- All throttle knobs: schedule, shards_per_pass, max_read_concurrency, fingerprint_batch_size

All 10 integration tests pass in p5_8_a_anti_entropy_fingerprint.rs.

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

notes/miroir-uhj.8.1.md

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+# P5.8.a: Anti-Entropy Fingerprint Step Verification
+
+## Bead: miroir-uhj.8.1
+
+### Summary
+
+Verified the P5.8.a Fingerprint step implementation (plan §13.8 step 1). The fingerprint functionality was already implemented in `crates/miroir-core/src/anti_entropy.rs`. All 10 integration tests pass.
+
+### Implementation Verified
+
+#### Core Fingerprint Logic (`AntiEntropyReconciler::fingerprint_shard`)
+
+Location: `crates/miroir-core/src/anti_entropy.rs:180-260`
+
+**Per-replica xxh3 digest:**
+- For each replica of a shard, iterates documents via `filter=_miroir_shard={id}` with pagination
+- For each document: computes `hash(primary_key || content_hash)`
+- Folds into a streaming xxh3 digest seeded by shard_id
+- Returns `ShardFingerprint` with merkle_root, document_count, and node_id
+
+**Canonical content hash (`compute_content_hash`):**
+- Excludes internal Miroir fields (`_miroir_*`, `_rankingScore`)
+- Serializes with sorted keys (via BTreeMap) for deterministic hashing
+- Uses xxh3 (XxHash64) for consistency with router
+
+**Self-throttling:**
+- 10ms sleep between batches to target <2% CPU
+- Configurable batch size via `fingerprint_batch_size` (default 1000)
+
+#### Throttle Knobs (AntiEntropyConfig)
+
+Location: `crates/miroir-core/src/anti_entropy.rs:22-48`
+
+- `schedule`: "every 6h" (parsed to seconds interval)
+- `shards_per_pass`: 0 = scan all shards
+- `max_read_concurrency`: 2 (reserved for future parallelism)
+- `fingerprint_batch_size`: 1000 documents per batch
+- `auto_repair`: true (enables repair on drift detection)
+
+### Tests Verified
+
+Location: `crates/miroir-proxy/tests/p5_8_a_anti_entropy_fingerprint.rs`
+
+All 10 tests pass:
+1. `test_fingerprint_shard_empty` - Empty shard handling
+2. `test_fingerprint_shard_single_document` - Single doc fingerprinting
+3. `test_fingerprint_shard_pagination` - Multi-batch pagination
+4. `test_fingerprint_shard_content_hash_excludes_internal_fields` - Canonical hash excludes `_miroir_*` fields
+5. `test_fingerprint_shard_different_content_different_hash` - Different content → different hash
+6. `test_fingerprint_shard_same_content_same_hash` - Same content → same hash
+7. `test_fingerprint_shard_key_order_independence` - JSON key order doesn't affect hash
+8. `test_fingerprint_shard_different_shard_ids_different_hashes` - Shard ID seeds the digest
+9. `test_fingerprint_config_batch_size` - Batch size configuration respected
+10. `test_compute_content_hash_unit` - Unit test for canonical hash
+
+### Integration Points
+
+- `AntiEntropyReconciler` in `anti_entropy.rs` - Core fingerprint logic
+- `AntiEntropyWorker` in `rebalancer_worker/anti_entropy_worker.rs` - Background worker with leader lease
+- `HttpNodeClient` - HTTP client for fetching documents from Meilisearch nodes
+- `Topology` - Shard-to-node assignment and node health checking
+
+### Files Modified
+
+No new implementation was required. The fingerprint step was already complete.
+- Tests were already passing (verified via `cargo test`)
+
+### Next Steps (P5.8.b, P5.8.c)
+
+- P5.8.b: Diff step - Compare fingerprints across replicas, identify divergent documents
+- P5.8.c: Repair step - Apply authoritative version to divergent replicas