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]

Thank you for your contribution. The zvec maintainer team is currently on holiday for the Chinese New Year (Spring Festival) in mainland China. Please expect delayed responses, and we appreciate your patience.

[kgeg401]

Added a quantized-path regression test for segment compaction:\n\n- New CompactTask_QuantizedVectorIndexThreeSegmentsRegression in segment_helper_test.cc.\n- Covers multi-segment, no-filter compaction with quantized vector indexes and verifies quantized index availability after compaction.\n\nThis is intended to directly exercise the get_quant_vector_indexer(...) merge-reuse path.

[kgeg401]

Added the quantized-path regression test for segment compaction, and the PR is otherwise unchanged. This should be ready for maintainer review when bandwidth allows.