feat: Indian lakh/crore grouping + locale-derived group sizes (v0.6)

[jlc488]

The last backlog item. The Indian system groups the last three digits and then every two — 12,34,56,789, not 123,456,789 — which the uniform regex grouper couldn't express. That's why it sat demand-gated; now it's requested.

Added

group accepts a [primary, secondary] pair:

format('100000',   { group: [3, 2] })  // '1,00,000'      1 lakh
format('10000000', { group: [3, 2] })  // '1,00,00,000'   1 crore

<input data-numkey data-numkey-group="3,2">

The regex grouper is replaced by a right-to-left loop covering both uniform and non-uniform sizes. A plain number still means uniform grouping (4 → 1234,5678), and group: 0 now disables separators instead of building a pathological \d{0} regex.

Fixed — a latent bug this exposed

locale only derived the separator and decimal mark, so a field with locale: 'en-IN' rendered 123,456,789: right separator, wrong grouping for that locale. localeSeparators now also reports group sizes — read out of Intl's formatToParts integer runs, which is how CLDR models primary/secondary — and resolveOptions applies them. Locales with ordinary thousands grouping are unaffected; an explicit group still wins.

Not affected

Caret math counts significant characters (digits), so it is grouping-agnostic by construction — no changes needed there, and a caret test under lakh grouping proves it.

Verification

• 12 new tests (112 total), including an exhaustive cross-check of both Indian and default grouping against Intl.NumberFormat across integer widths 1–15 — 0 mismatches.
• Real browser, char-by-char (this repo has a history of DOM bugs that whole-value tests missed): 1 → 1,234 → 12,345 → 1,23,456 → 12,34,56,789 with the caret tracking correctly at every step, plus a mid-string insertion (1,02,34,56,789, caret after the inserted digit).
• Demo site locale switcher gains en-IN; verified rendering 12,34,567.89 while the canonical value stays "1234567.89".
• Docs: new section in both READMEs, attribute + option tables, roadmap marks v0.6 shipped (backlog now empty).