refactor(spec): step 18's rationale as key-sorted fragments, its conversionIds derived

Every major-18 retirement appended to two tails of `step18` in
migrations/registry.ts: the `+`-chained rationale (rewriting its closing
line) and the `conversionIds` list. Two retirements in flight therefore
conflicted in GitHub's driver-free merge. A plain array appended at its end
does not fix that: git conflicts on any two insertions into the same gap.

The rationale is now 46 fragments `{ id, order, text }` kept SORTED BY KEY,
so two retirements insert at different lines, and rendered by `order`
(joined with one space). `conversionIds` is read off
`CONVERSIONS_BY_MAJOR[18]`, of which it was a value-identical copy.

The rendered rationale, the conversion ids, the chain hop and the whole
MIGRATIONS_BY_MAJOR value are byte-identical to the parent.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014EJ1ED8X4MMrT18BhVx4tx

Author: claude