fix: optimize segment compaction by reusing first vector index

[kgeg401]

Summary

• add a no-filter compaction fast path that reuses the first segment's vector index as merge base
• keep full rebuild behavior as fallback for filtered compaction and unsupported index layouts
• cover both vector-index and filtered fallback regressions with segment helper tests

Root Cause

ReduceVectorIndex always rebuilt merged indexes from scratch, even when compaction had no filter and segments were layout-compatible. That caused avoidable index rebuild work during merge/compact.

Validation

• Added CompactTask_VectorIndexThreeSegmentsRegression to verify no-filter multi-segment compaction correctness
• Added CompactTask_FilterMultiSegmentsRegression to verify filtered path still uses fallback semantics
• Local compile/test execution is not available in this environment (no C/C++ compiler toolchain), so CI is the authoritative validation

Fixes #98

[feihongxu0824]

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