fix: close remaining sharding correctness holes

[renecannao]

Summary

Stacked on #58.

• Multi-table DML: fail closed if any table is sharded or backends differ.
• HAVING: rewrite COUNT(*) / other aggs to aggregate-output columns.
• Co-located joins: same shard layout + equi-join on each table's shard key → per-shard JOIN SQL.
• 2PC in tools: backend mode uses DistributedTransactionManager; in-txn SELECTs use pinned sessions. sqlengine --txn-log PATH optional.

Validation

• ./run_tests --gtest_brief=1: 1320 passed, 38 skipped
• ./run_tests --gtest_filter='PlanExecutorTest.Having*:DistributedPlannerTest.Having*:DistributedPlannerTest.Colocated*:DistributedDmlTest.MultiTable*'
• sqlengine.cpp and mysql_server.cpp compile

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