fix(local-fields): cascade remove on group delete

[faisalahammad]

acf_remove_local_field_group() removed the group from local-groups but left its fields orphaned in the local-fields store. Orphaned fields remained discoverable by key/name lookups after their parent group was gone.

New _acf_cascade_remove_local_fields() helper recursively queries by parent key and removes each child field, including nested sub-fields under repeaters. Both the data entry and name alias are cleaned up per field.

Closes #458

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[faisalahammad]

CI check note — pre-existing WP trunk flake

The only failing CI check on this PR is E2E - WP trunk / Shard 4/4 (run 30203543529, job 89797567940). It does NOT appear linked to this PR:

• PR branches only touch includes/local-fields.php (+34/-1) and tests/php/includes/test-local-fields.php (+71/-11). No .spec.ts, JS, or admin-tools code was changed.
• All other CI checks pass: phpcs, PHPStan, PHP 7.4, PHP 8.4, Node 20, E2E WP 6.9 (all 4 shards), E2E WP latest (all 4 shards), E2E WP trunk (shards 1-3/4).
• The failing tests (should update existing post type / taxonomy / options page on import with same key in tests/e2e/import-export.spec.ts at lines 1398/1534/1666, plus bulk-action checkbox timeouts in post-type-admin.spec.ts around line 664) live in WP's import and bulk-action admin UI — code paths this PR does not touch.
• Local reproduction on this branch against WP 7.0.2 (current latest, the version wp-env serves by default) shows all 3 Import Overwrite tests pass.

The cascade helper _acf_cascade_remove_local_fields() runs only inside acf_remove_local_field_group(), which is called from teardown in PHP tests only. The import flow (admin-tools.php → acf_import_internal_post_type → update_post) never invokes it.

Recommendation: treat E2E - WP trunk / Shard 4/4 as pre-existing WP-trunk-specific flake, unrelated to this PR. Worth tracking separately if it recurs on other PRs.