fix(spec): the sortField retirement's step-18 rationale fragment takes order 70

#21622 (#21464 stage 4) landed on main as 3f1bc81 with its step-18
rationale fragment `ui-object-metric-aggregate-trend-typed` at order 69,
the order this branch's fragment also held. Re-read on the merged tree's
main side (5b5e83f) the highest STEP18_RATIONALE order is 69, so this
fragment takes the next free one, 70, and now renders after #21622's.
The D2 conversion keeps order 60: the highest MAJOR_18_CONVERSIONS order
on main is still 59 (`elementTextVariantHeadingLevels`).

Claude-Session: https://claude.ai/code/session_01T9u38rswFp5Rw8DswRUReJ
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

packages/spec/src/migrations/registry.ts

@@ -5854,7 +5854,7 @@ const STEP18_RATIONALE: readonly RationaleFragment[] = [
   },
   {
     id: 'object-master-detail-form-detail-sort-field-retired',
-    order: 69,
+    order: 70,
     text:
       'It also retires an `object-master-detail-form` detail entry\'s `sortField` (#21589, ADR-0049 '
       + 'enforce-or-remove; the spec half of objectui#11070 round 9). The console stopped reading the '