feat: reserve dense map value storage

[hurricane1026]

Summary

• add an explicit reserve_values() API for flat and indirect dense-map storage
• allow the contiguous value array to grow directly to a requested minimum capacity
• verify reserved string-map values do not move while filling the reservation

This keeps the existing memory-saving behavior of reserve() unchanged while allowing reducers that know their final cardinality to avoid repeated value-array reallocations and moves.

Tests

• containa_test: 309/309 test cases passed (1,595,223 assertions)
• ClapDB ClickBench Q34, 100M rows, 32 cores, 10 tries: hot median 1.677s vs 1.682s baseline