feat: prune OR/range, pool PostgreSQL, 2PC stress, honest docs

[renecannao]

Summary

Stacked on #56 (which now contains #57–#59).

• Prune:id = 1 OR id = 2 unions shards; RANGE id <= N / BETWEEN prunes. id = ? still scatters.
• PG pool:ThreadSafeMultiRemoteExecutor dispatches PostgreSQL through PgConnectionPool + pinned PooledPgSession.
• Stress:engine_stress_test uses DistributedTransactionManager.
• Docs: README add_table (not add_sharded_table); Session example uses 2PC; pooling/DML claims match the code.

Validation

• ./run_tests --gtest_brief=1: 1325 passed, 39 skipped
• ./run_tests --gtest_filter='DistributedPlannerTest.ShardRouting_*:ShardMapRangeTest.*'

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