OP#3: Add completion notes for resharding vs scaling documentation

Add notes/bf-5xs1.md documenting the completion of OP#3 work.

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

notes/bf-5xs1.md

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+# OP#3: Resharding (S-change) vs. Node Scaling (N-change) - COMPLETED
+
+## Task Summary
+
+Completed the remaining work for Open Problem #3 by documenting the trade-offs between S-change (resharding) and N-change (node scaling).
+
+## What Already Existed
+
+The codebase already had excellent implementation and validation:
+
+1. **Benchmark resharding with realistic corpora** - `docs/benchmarks/resharding-load.md`
+   - Comprehensive test matrix covering small/medium/large documents
+   - Empirically validates the 2× transient storage and write load caveat
+   - All scenarios confirm exactly 2.0× storage and dual-write amplification
+
+2. **CLI schedule guidance for off-peak windows** - `crates/miroir-ctl/src/commands/reshard.rs`
+   - Schedule window guard with time-based restrictions
+   - `--schedule-window` flag to specify allowed windows
+   - `--force` override with warnings
+   - Dry run support
+
+3. **Core resharding implementation** - `crates/miroir-core/src/reshard.rs`
+   - Six-phase shadow-index operation (§13.1)
+   - Configuration management
+   - Load simulation and validation
+
+## What Was Added
+
+Added comprehensive "Resharding (S-Change) vs Node Scaling (N-Change)" section to `docs/trade-offs.md`:
+
+- **Core distinction table**: Compares N-change (node scaling) vs S-change (resharding)
+- **Node scaling explanation**: Lightweight operation using rendezvous hashing, moves only ~1/N of documents
+- **Resharding explanation**: Heavy operation with 2× transient storage/write amplification
+- **Decision matrix**: Helps operators choose the right approach based on symptoms
+- **Capacity planning guidance**: S = max_nodes_per_group_ever × 8 formula with rationale
+- **Operator guidance**: Steps to follow if resharding is necessary
+
+## Key Insights
+
+The main insight for operators: **Node scaling is lightweight; resharding is heavy.**
+
+- Adding nodes: only ~1/N of documents move (those whose top-ranked node changes)
+- Resharding: every document's shard assignment changes, requiring full dual-hash dual-write
+- Prefer N-change over S-change whenever possible
+- Choose S generously at index creation to avoid ever needing to reshard
+
+## Commit
+
+```
+commit 1fa5187
+OP#3: Document S-change (resharding) vs N-change (node scaling) trade-offs
+```
+
+## Status
+
+**COMPLETED** - All OP#3 requirements addressed:
+- ✅ Benchmark resharding operations with realistic document distributions
+- ✅ Validate transient storage and write load multiplier assumptions
+- ✅ Add CLI schedule guidance for off-peak reshard windows
+- ✅ Document trade-offs between S-change (resharding) and N-change (node scaling)