feat: add captureGeo opt-in for region/city/lat/lng/timezone

[Gdewilde]

Summary

• Adds a `captureGeo` opt-in to `trackVisit`. When enabled, attaches `region`, `city`, `latitude`, `longitude`, and `timezone` derived from Vercel's `x-vercel-ip-*` edge headers.
• City and region are URL-decoded (Vercel encodes them, e.g. `San%20Francisco`). Missing headers are omitted rather than emitted as empty strings.
• Off by default — city-resolution geo is more identifying than the country-only `captureCountry` flag, so opt in deliberately.
• Bumps version to 0.9.0.

This pulls the `geoProperties` helper out of apideck-io/developer-docs#798 and into the library so other Vercel-hosted consumers don't have to reinvent it. Once this lands, that PR can drop the helper and just set `captureGeo: true`.

Test plan

• `npm run typecheck && npm test` — 110 tests pass, including 3 new ones covering decode, missing-header omission, and the off-by-default behavior.
• After merge, confirm auto-release + auto-publish flow (fixed in fix(ci): explicitly dispatch publish.yml after creating release #10) ships 0.9.0 to npm without manual intervention.
• Update apideck-io/developer-docs#798 to drop the local `geoProperties` helper and set `captureGeo: true` on the `trackVisit` call.

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