docs(skills): date-bucket engine sentences name what each arm emits

The dashboards rule said Postgres buckets with date_trunc and MongoDB with
$dateTrunc; the drivers emit a label (to_char / date_format / strftime /
$dateToString, in memory bucketDateKey), never an instant. The 'week' row
now reads the ISO week label YYYY-Www every arm answers, and the
aggregation rule's push-down sentence names the same expression family
instead of DATE_TRUNC.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CB6W87z22K2yjUCDyVrJRk

Author: claude

skills/objectstack-query/rules/aggregation.md

@@ -79,10 +79,10 @@ aggregations (never bucket by hand in app code):
   the projected COLUMN only** — grouping still keys on the field, so the buckets
   themselves are unchanged. Read the result under `alias ?? field`, and reference
   that same name from `having`.
-- The engine pushes bucketing down to the driver (`DATE_TRUNC` etc.) when
-  the dialect supports that granularity, and transparently falls back to
-  in-memory bucketing otherwise — results are correct either way, **including
-  the column keys**.
+- The engine pushes bucketing down to the driver (`to_char` / `date_format` /
+  `strftime` / `$dateToString`, never `date_trunc`) when the dialect supports
+  that granularity, and transparently falls back to in-memory bucketing
+  otherwise — results are correct either way, **including the column keys**.
 
 ## HAVING Clause
 

skills/objectstack-ui/rules/dashboards.md

@@ -318,14 +318,14 @@ const signedByMonth: DashboardWidget = { id: 'signed_by_month', type: 'line',
 | `dateGranularity` | Rendered bucket label |
 |:--|:--|
 | `'day'` | `YYYY-MM-DD` |
-| `'week'` | ISO date of the bucket (`YYYY-MM-DD`) |
+| `'week'` | ISO week `YYYY-Www` |
 | `'month'` | `YYYY-MM` |
 | `'quarter'` | `YYYY-Qn` |
 | `'year'` | `YYYY` |
 
-* **Engine support** — Postgres `date_trunc`, MySQL `date_format`, SQLite
-  `strftime`, MongoDB `$dateTrunc`, in-memory fallback. All emitted by the
-  analytics service, not the client.
+* **Engine support** — drivers emit the bucket as a label (`2026-01`), not
+  an instant: Postgres `to_char`, MySQL `date_format`, SQLite `strftime`,
+  MongoDB `$dateToString`, in-memory `bucketDateKey`.
 * **Human labels are automatic** — the analytics layer formats the bucket value
   to the label above, and resolves `select`/`lookup` dimension values to their
   option label / related-record name. Measures carry their `label` + `format`