fix(2meet-data-optimizer): join each routed group and drop the redundant GROUP BY

- posts_join joins the flat table of every routed group (a wp_core key next to a post type group no longer fails with "Unknown column")
- Flat tables are one row per post (UNIQUE post_id): remove the posts_groupby hook; skip the INNER JOIN status/date copy route for NOT EXISTS, which must match posts without a flat row
- Verify: tests/live/run.php router (9/9); qbench Q05 475 == native, no SQL error; price filter EXPLAIN has no Using temporary
- PR: PR-061

tests/live/README.md

@@ -39,7 +39,7 @@ Exit code 0 when every test passed, 1 otherwise. Output per test:
 | `shadow_write` | writes in shadow_read reach native meta (user, term, HivePress listing) |
 | `serialized` | array meta values are stored serialized in flat tables, never "Array" |
 | `resync` | dual_write / shadow_read: flat follows native after every write (4 entities × 2 modes: update, same value, delete, refused writes, stale flat, key rename by meta_id) |
-| `router` | a routed WP_Query returns the same ids, in order, as native (OR, several post types, untyped numeric compares, named orderby, LIKE) |
+| `router` | a routed WP_Query returns the same ids, in order, as native (OR, several post types, untyped numeric compares, named orderby, LIKE, keys of two groups, NOT EXISTS with the status/date copy route); routed SQL has no GROUP BY |
 | `zero_side_effect` | zone tables that were never created are never queried on save / delete |
 
 ## Tools (`tools/`)