fix(release): re-date a pre-written current-series field on its GA's own version run

sync-release-index-currency left a same-version status field alone on
every run, date included. That is right for prerelease cuts (the 17.7.0
re-dating its header records), but on the run whose `changeset version`
writes a GA's CHANGELOG heading the run's date is the true one, and a
field written by hand ahead of the cut is not: 18.0.0 shipped dated
2026-10-19 while its version pass ran and npm published on 2026-10-20.

The run is detected without a clock or the network, per major: the
newest GA's heading is in the working tree's spec CHANGELOG and not in
the one committed at HEAD (read with git show, GIT_* stripped). Only
then is a same-version field re-dated. Prerelease runs, runs with
nothing to version and runs after the version commit landed keep the
refusal. An unreadable HEAD is a refusal, and rewriteIndexText now
requires the HEAD CHANGELOG instead of defaulting either way.

Self-test: new battery Control I (16 cases, floor 9 batteries); Control
H's end-to-end tree is now a real throwaway repository.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013SFSGWz48S21trBAfXEXUx

Author: claude