docs: lock fixed_size ±3 in decision record and lld

Why:
- Close the open ±N follow-up from the strategy finalization memo.

Changes:
- Add addendum with ±0…±3 evidence and binding ±3/3 default.
- Sync LLD config table with new neighbor expansion defaults.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/LLD.md

@@ -84,7 +84,7 @@ get_qdrant_client()
 | `ollama_base_url` | host URL | Local embeds |
 | `qdrant_url` | `http://localhost:6333` | Vectors |
 | `top_k` | `5` | Retrieval |
-| `neighbor_prev` / `neighbor_next` | `0` / `0` | Expansion defaults |
+| `neighbor_prev` / `neighbor_next` | `3` / `3` | Expansion defaults (fixed_size ±3) |
 | `chunk_size` / `chunk_overlap` | `512` / `50` | fixed_size / recursive targets |
 | `semantic_threshold` | `0.3` | Fallback if model default missing |
 | `semantic_min_chunk_size` | `3` | Min units per semantic chunk |