[Goldloli]

Fixes #383

Description

A ticking format="duration" / format="elapsed" element visibly jumps around as it counts, because proportional digits change width between ticks. The rendered span[part="root"] now gets font-variant-numeric: tabular-nums for those two formats, following the write-skipping pattern of #updateRenderRootContent — the inline style is only written when its value actually changes, so the periodic-tick fast path is untouched.

• Other formats get no inline style at all, exactly as before.
• The default stays overridable: ::part(root) { font-variant-numeric: normal !important } (an author !important rule beats a non-important inline style).

Of the two shapes discussed in the issue (document a user-side rule vs make it the default), this makes it the default for the ticking formats only — the timer jumping is visible out of the box, while date/relative typography is unchanged.

Verification

• npx web-test-runner — 640 passed, 0 failed, including two new tests: one asserts duration and elapsed render with tabular-nums and another asserts other formats get no inline numeric styling; without the change the first fails with expected '' to equal 'tabular-nums'
• npx tsc --noEmit and npx eslint src/relative-time-element.ts test/relative-time.js — clean