Paginated members list on segment edit page #7412

Scope

Replace the current list of identities in the 'Identities' tab within a segment to show only the identities that belong to the segment. Below the select of environment:

• A list of identifiers matching the segment in the selected env, paginated via the identifier cursor.

Acceptance criteria

• Selecting an env populates the listing within p95 < 1s on a 10M-identity environment.
• Listing handles empty state ("0 matching identities").
• A new FE flag to match the API flag.
• If the feature flag is disabled, show the legacy sampling list.
• One Playwright e2e test covering: select env → assert listing → click "next page" → assert next page renders.