P2.2: Verify write path implementation - all acceptance tests pass

Verified the write path implementation in crates/miroir-proxy/src/routes/documents.rs:
- POST /indexes/{uid}/documents - Add documents
- PUT /indexes/{uid}/documents - Replace documents
- DELETE /indexes/{uid}/documents/{id} - Delete single document by ID
- DELETE /indexes/{uid}/documents - Delete by IDs array or filter

All acceptance criteria satisfied:
- Primary key extraction on the hot path
- _miroir_shard injection into every document
- Reserved field rejection (_miroir_shard, _miroir_updated_at, _miroir_expires_at)
- Two-rule quorum (per-group quorum = floor(RF/2) + 1)
- Per-batch grouping for efficient fan-out
- Delete-by-filter broadcast to all nodes
- Delete-by-IDs array with independent per-shard routing

Test results:
- 11/11 acceptance tests pass (tests/p22_write_path_acceptance.rs)
- 18/18 unit tests pass (routes/documents.rs)
- 15/15 integration tests pass (tests/p22_write_path.rs)

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

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