feat: parallel scatter, composite keys, LIST prune, plan-cache redistribute

[renecannao]

Summary

• Flatten UNION ALL remotes so N shards open in parallel, not just the innermost pair.
• Composite HASH keys (col1+col2), fail-closed unknown tables, and shard-key UPDATE that moves the row.
• LIST misses no longer route to shard 0; BETWEEN/IN prune mapped values. Composite colocated joins push when every key part is equated.
• Plan cache stores the logical plan and re-distributes on each hit so a later ShardMap change is not stale.

Stacked on #60 (fix/shard-prune-pg-docs).

Test plan

• ./run_tests --gtest_brief=1 — 1341 passed, 39 skipped
• New coverage: n-ary UNION ALL, composite INSERT/SELECT/join, LIST miss + BETWEEN, plan-cache remap

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