SQL differential fuzzer: PR gate runs 2 of 15 groups, and no group generates implicit rowids or AUTOINCREMENT

[timsehn]

Summary

1. test/sql_differential_test.sh:66-75: DOLTLITE_DIFF_GROUPS=default expands to --include-large-ints --include-desc only, and test.yml:311-317 runs seeds 1..10000 with that. The other 13 groups (expr agg setops cte window joins writesel ddl constraints triggers returning generated fkeys) run only in nightly-fuzz.yml:238-242, where a divergence opens an issue instead of blocking. A regression in trigger/FK/generated/RETURNING/DDL semantics can merge and surface a day later (classes: A VIRTUAL column hides a drop-versus-update merge conflict #3097 generated, ALTER TABLE DROP COLUMN with two indexes on the column names a different index in its error than stock #3045 ddl, DELETE leaves an orphaned index entry when the entry's key no longer matches the row #2922 constraints/writesel).
2. test/sql_differential_fuzzer.py:205,284,288,306: every INSERT names the key column; schema() (96-136) never emits AUTOINCREMENT; there is no last_insert_rowid() read, and ROLLBACK TO (466) is never followed by an implicit-rowid insert. The rowid bugs fixed recently (ROLLBACK TO a savepoint burns rowids #3098, A sqlite_sequence seed moves non-AUTOINCREMENT rowids #3099, Implicit-rowid INSERT returns SQLITE_FULL instead of the random-rowid fallback #2908) are unreachable by construction.

Proposal

Add a rowid group (implicit-rowid INSERTs, INTEGER PRIMARY KEY AUTOINCREMENT, last_insert_rowid(), sqlite_sequence reads/writes, ROLLBACK TO / RELEASE followed by implicit inserts, keyless INSERT … SELECT) and include it in the PR default. Rotate a subset of the other 13 groups through the PR gate by seed so each gets PR coverage within the existing time budget.

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