Map source tab interfaces to WordPress 7.1 core Tabs blocks

## Problem

The fixture corpus contains recurring accessible tab structures that currently route through generic/runtime preservation. WordPress 7.1 stabilizes `core/tabs`, `core/tab-list`, `core/tab-panels`, and `core/tab-panel`, so this is now a native transformer path rather than a companion-block candidate.

References:
- WordPress 7.1 roadmap: https://make.wordpress.org/core/2026/06/19/roadmap-to-7-1/
- Gutenberg stabilization: https://github.com/WordPress/gutenberg/pull/80163
- Tracking issue: https://github.com/WordPress/gutenberg/issues/73230

Representative fixtures:
- `7-event-conference/schedule.html`: ARIA tablist and conference-day panels.
- `33-sports-team-league/leaderboard.html`: tabbed leaderboard table groups.
- `83-startup-docs-hub/index.html`: tabbed code examples.

## Contract

- Recognize tablist/tab/panel relationships structurally and through ARIA IDs.
- Emit the WordPress 7.1 core Tabs family with synchronized labels, order, initial selection, and editable panel inner blocks.
- Preserve author CSS through exact source provenance without carrying source tab JS.
- Gate the mapping on runtime block availability/version; do not invent a parallel companion block.
- Verify editor validity, keyboard/ARIA behavior, feature parity, and visual parity on WordPress 7.1.

Start with the representative fixtures: 7-event-conference/schedule.html, 33-sports-team-league/leaderboard.html, and 83-startup-docs-hub/index.html, comparing their tablist, tab, panel, and ARIA ID relationships. Done means the transformer emits the WordPress 7.1 core Tabs family only when the runtime blocks are available, preserves exact source CSS provenance without source tab JavaScript, and passes editor, keyboard/ARIA, feature, and visual parity checks.

Written by the indexing model from the issue text.