fix: CMake export, C API exports, drop/create scalar index, release CI

[ghshhf]

Summary

This PR fixes multiple issues and adds CI improvements.

Changes

Fix #468: Add CMake package export support

• Add install(EXPORT ...) to CMakeLists.txt (root, src, src/binding/c)
• Create cmake/zvecConfig.cmake.in template
• Users can now find_package(zvec) and find_package(zvec COMPONENTS c_api)

Fix #427: Support DROP + CREATE scalar index

• Modify src/db/index/segment/segment.cc to allow recreation of scalar index after drop
• Add tests: Feature_DropAndRecreateScalarIndex, Feature_DropAndRecreateScalarIndex_MultipleFields

Fix #443: Export missing C API functions

• Add standalone Reranker C API: zvec_reranker_create_rrf(), zvec_reranker_create_weighted(), zvec_reranker_destroy()
• Add zvec_multi_query_set_reranker()
• Add zvec_doc_set_sparse_vector()
• All functions have ZVEC_EXPORT and proper opaque pointer types

Add #390: Release pre-built libraries CI workflow

• Add .github/workflows/09-release-build.yml
• Multi-platform matrix build (shared + static libraries):
  • Ubuntu 20.04/22.04/24.04 (x64), Ubuntu 22.04 (arm64)
  • macOS 13 (Intel x86_64), macOS 14 (Apple Silicon arm64)
  • Windows 2022/2025 (x64)
  • Fedora 39/40/41 (via Docker container)
• Trigger: push tags v* or manual dispatch
• Optional GitHub Release publishing

Test plan

• CMake package export works (find_package(zvec))
• C API functions link correctly
• Drop + create scalar index round-trip passes
• CI workflow tested on fork (release-build.yml)

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