perf(memory): introduce string interning pool for low-cardinality query results (#604)

[ZhuchkaTriplesix]

Summary

Introduces a high-performance, bounded string interning pool (StringInternPool) during query result decoding in result_row_string_convert.dart to deduplicate repeated cell strings across low-cardinality columns.

Changes

1. StringInternPool (lib/core/database/result_row_string_convert.dart):
   • Preloaded common database literals (NULL, true, false, 0..10, active, inactive, pending, etc.).
   • Deduplicates repeated cell strings up to a configurable maximum length (default 128 chars) and pool size (default 4096 entries).
2. Batch & Yielding Decoders:
   • Integrated StringInternPool into convertResultRowsToStringsSync, convertResultRowsToStringsYielding, and convertResultRowsToStringsAdaptive.
3. Tests & Benchmarks:
   • Unit tests in test/core/database/result_row_string_convert_test.dart verifying pointer identity reuse on duplicate cell values and bounds.
   • Benchmark test validating sub-100ms decoding on 10,000 low-cardinality rows with pointer reuse across columns.

Closes #604