[C#] Update RawTableIterBase.Enumerator to use ArrayPool for buffer

[rekhoff]

This is the implementation of a fix for #4093

Description of Changes

• Updated RawTableIterBase.Enumerator to rent its scratch buffer from ArrayPool<byte>.Shared (with dynamic re-rent on BUFFER_TOO_SMALL) and return it on dispose, so iterating rows no longer allocates a fresh byte[] per step.
• The enumerator still exposes the row bytes via ArraySegment<byte> Current, but now the underlying storage is reused across iterations rather than recreated each time.

Testing results from master:

allocated_bytes=14000
elapsed_ticks=1521829
sum=1249975000
row_count=50000
Find() TinyRecord (8 bytes payload) -> 132208 bytes allocated
Find() MediumRecord (~50 bytes payload) -> 132440 bytes allocated
Find() LargeRecord (1 KB payload) -> 134528 bytes allocated
Find() LargeRecord (10 KB payload) -> 152408 bytes allocated
Find() LargeRecord (100 KB payload) -> 336728 bytes allocated
Iter() 10 rows -> 131896 bytes allocated
Filter() iterate 20 rows -> 133288 bytes allocated
Filter() iterate 100 rows -> 135976 bytes allocated
10x consecutive Find() (TinyRecord) -> 1319120 bytes allocated

Testing results with this fix:

allocated_bytes=14000
elapsed_ticks=1504949
sum=1249975000
row_count=50000
Find() TinyRecord (8 bytes payload) -> 1096 bytes allocated
Find() MediumRecord (~50 bytes payload) -> 1280 bytes allocated
Find() LargeRecord (1 KB payload) -> 4464 bytes allocated
Find() LargeRecord (10 KB payload) -> 27464 bytes allocated
Find() LargeRecord (100 KB payload) -> 234312 bytes allocated
Iter() 10 rows -> 680 bytes allocated
Filter() iterate 20 rows -> 1872 bytes allocated
Filter() iterate 100 rows -> 3280 bytes allocated
10x consecutive Find() (TinyRecord) -> 8000 bytes allocated

API and ABI breaking changes

No API or ABI changes

Expected complexity level and risk

2 - low/moderate:
Touches the hot-path iterator that every Iter/Find/Filter call uses

Testing

• Compiled CLI and ran regression tests locally
• Verified iterator-based harness runs (client + module reducers) on both master and this branch, confirming allocations drop from ~131 KB per read to payload-scaled values.
• Ensured no regressions in standard harness sanity checks (row_count=50000, sum=1249975000).