Add MIGRATION_NOTES.md: doc_id formula change 2026-03-13

MIGRATION_NOTES.md

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+# Migration Notes
+
+## doc_id formula change — 2026-03-13
+
+**Changed:** `doc_id` is now computed as `UUID5(NamespaceURL, url)` — a deterministic UUID derived from the canonical URL, stable across crawls and reruns.
+
+**Before (shards below):** `doc_id` was the UUID extracted from the WARC-Record-ID header:
+```
+doc_id = TrimPrefix(warc_record_id, "<urn:uuid:")   # e.g. <urn:uuid:abc-123> → abc-123
+```
+
+### Affected shards — CC-MAIN-2026-08 (232 shards, need re-export)
+
+| Range | Count |
+|---|---|
+| 00000–00047 | 48 |
+| 00058 | 1 |
+| 00101–00148 | 48 |
+| 00251–00291 | 41 |
+| 00401–00424 | 24 |
+| 00551–00562 | 12 |
+| 00701–00720 | 20 |
+| 00851–00888 | 38 |
+| **Total** | **232** |
+
+These shards will be re-exported automatically as the pipeline processes them again with `--republish`.
+
+### Client-side workaround (no re-export needed)
+
+The `url` column is correct and stable in **all** shards. Recompute `doc_id` client-side:
+
+```python
+import uuid
+doc_id = str(uuid.uuid5(uuid.NAMESPACE_URL, row["url"]))
+```
+
+```go
+import "github.com/google/uuid"
+docID := uuid.NewSHA1(uuid.NameSpaceURL, []byte(row.URL)).String()
+```