[seekdb][memory] Reduce small-object memory overhead

[hnwyllmm]

Task Description

SeekDB lite uses only a very small set of runtime objects in single-node scenarios, but several inherited OceanBase containers still reserve large default bucket arrays or per-thread allocator structures. Those small but always-resident allocations add up in the sub-1MB memory classes.

Solution Description

Shrink low-cardinality hash map/tablet/stat/DDL buckets for seekdb-lite usage, reduce MDS/TX allocator concurrency, cap ParamCharsType SEArray block size, and remove unused Oracle NUMBER cast boundary arrays. MemDumpContext is intentionally excluded because it is handled separately.

Passed Regressions

• ./build.sh release --init --ob-make
• ob-make
• Local seekdb smoke test for DDL/DML/DECIMAL/multi-parameter query
• Local 300-table smoke test to verify smaller bucket counts are not capacity limits
• Runtime memory check through oceanbase.v$ob_memory; directly attributable reduction is about 4.63 MiB

Upgrade Compatibility

No storage format, protocol, SQL syntax, or upgrade compatibility impact expected.

Other Information

MR contains one commit: 0fde6e1f7ac Reduce small-object memory overhead.

Release Note

[hnwyllmm]

The mapping Dima issue is about optimizing small memory blocks.