fix(spec): step 18's migration rationale states each cited decision in words, not tracker numbers

The 49 STEP18_RATIONALE fragments that carried a tracker id (88 ids: 72
objectstack, 12 objectui, 4 decision-batch numbers) now say what the cited
record decided, or drop a citation the sentence already explained. Text
only: fragment ids, orders, entries, conversions and schemas are unchanged.

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

Author: claude