Use compact primitive storage for star-tree record offsets in OffHeapSingleTreeBuilder

[Jackie-Jiang]

Summary

OffHeapSingleTreeBuilder keeps the star-tree record file offsets in a List<Long>, which costs ~28 bytes of heap per record (24-byte boxed Long plus a 4-byte reference under compressed oops). The number of star-tree records can go into the hundreds of millions for large segments with a wide dimensionsSplitOrder; in one production incident the offsets list alone held ~700M boxed Longs (~19.6GB including the ArrayList backing array), OOMing the server during realtime segment conversion.

This PR replaces the boxed list with a compact RecordOffsets structure:

• Offsets are tracked as a prefix sum of the appended record lengths, so appendRecord passes the record length instead of computing the next offset from a read-back of the last element.
• Start offsets are stored in an IntArrayList (4 bytes per record) until the record file grows beyond Integer.MAX_VALUE, and in a LongArrayList (8 bytes per record) afterwards. Offsets increase monotonically, so the switch happens at most once and needs no copy: reads pick the list by comparing the index against the int head's size.
• The end offset (total file length) is a plain long field, removing the extra end-sentinel entry and the boxing/unboxing on every append and on every read in the star-node sort comparator.

For the build above this reduces the offset bookkeeping from ~19.6GB to ~5.6GB of heap (4 bytes per record while the record file stays under 2GB), and removes the Long allocation churn from the build hot path.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.12%. Comparing base (11c3429) to head (bd52feb).

Files with missing lines	Patch %	Lines
.../startree/v2/builder/OffHeapSingleTreeBuilder.java	91.30%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19317      +/-   ##
============================================
- Coverage     67.12%   67.12%   -0.01%     
  Complexity     1424     1424              
============================================
  Files          3462     3462              
  Lines        220669   220677       +8     
  Branches      35253    35255       +2     
============================================
- Hits         148129   148123       -6     
- Misses        60720    60722       +2     
- Partials      11820    11832      +12     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	67.12% <91.30%> (-0.01%)	⬇️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.12% <91.30%> (-0.01%)	⬇️
unittests	67.11% <91.30%> (-0.01%)	⬇️
unittests1	57.79% <78.26%> (-0.04%)	⬇️
unittests2	39.13% <91.30%> (+0.01%)	⬆️

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