Add Voters duplicate-check probe

Reproduces #1513 without a browser.

add_roll_repro.mjs transcribes three things verbatim from packages/frontend/src/components/Election/Admin/AddElectionRoll.tsx at Equal-Vote/bettervoting main @ 7bc75a82 — the row-building loop out of onSubmit (:55–:89), duplicatesExist (:173) and removeDuplicates (:158). Only the React edges are stubbed (setSnack, confirm, postRoll); confirm() is answered by the caller so both the YES and NO branches can be exercised.

node add_roll_repro.mjs

Recorded output: run.out. It shows that in admin-managed-voter-ID mode any submission of 2+ rows prompts, YES posts only the first row, and NO posts nothing — while the email mode it was written for behaves correctly. The last block replays the reporter's session and lands on the roll counts visible in their screen recording: 2 → 3 → 4 → 5 for submissions of 3, 2 and 1 rows.

What this does not prove: that the component calls these functions the way the file reads. That wiring is still source-read — see ../../issues/add-voters-duplicate-check-keys-on-email.md and the click-through cases BV250a / BV250b.

Method notes: ../../reference/automation-gotchas.md §6.