fix: distribute derived tables and stop silent sort/SQL bugs

[renecannao]

Summary

Stacked on #57.

• FROM (SELECT …):distribute_node rewrites DERIVED_SCAN.inner_plan. Inner scans become REMOTE_SCANs.
• remote_sql_len:uint16_t → uint32_t (matches StringRef). Test uses a 70k literal.
• MERGE_SORT keys:ORDER BY position/alias rewritten to the select-list expr in PlanBuilder. Expression keys no longer merge on column 0 — they gather and sort locally.

Validation

• ./run_tests --gtest_brief=1: 1315 passed, 38 skipped
• ./run_tests --gtest_filter='DistributedPlannerTest.Derived*:DistributedPlannerTest.OrderBy*:DistributedPlannerTest.RemoteSql*:PlanExecutorTest.OrderBy*'

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