Closes the silent-skip hole reviewers flagged: with the index/count
hardcoded in three sibling strings, a copy-paste slip (shard-2of3 running
--shard=1/3) or a partial 3->4 migration would silently skip a third of
the 428-file suite while CI stays green.
scripts/run-ui-shard.mjs parses N/M from npm_lifecycle_event (the script
NAME is the single source of truth), validates the package's shard family
is exactly 1..M for one M, and delegates through 'npm run test:ui' so the
vitest command stays single-sourced.
Mutation-verified: shard-9of3 name -> exit 1 'index out of range';
adding shard-4of4 beside the 3-family -> exit 1 'must form exactly 1..M';
correct invocation runs shard 2/3 (142 files) identically to before.
kshitij
a66a643863