feat: composite RANGE, semi-join prune, PG LIST/RANGE + 2PC demo

[renecannao]

Summary

• Composite RANGE now routes and prunes on the first key component (LIST still rejects composites).
• Semi-join prune: when one side of a join is sharded and the other is not, the engine materializes the small side and pushes an IN (...) onto the sharded probe.
• New PostgreSQL sharding demo (scripts/start_pg_sharding_demo.sh + run_pg_sharding_demo.sh) on ports 16432/16433 exercising RANGE + LIST + cross-shard 2PC.
• sqlengine now auto-selects POSTGRESQL 2PC dialect when all backends are pgsql://.
• Added planner tests + live PG backend test (skips if containers are not running).

Stacked on #61.

Test results

./run_tests --gtest_brief=1 → 1344 passed, 40 skipped

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