Guard the SVS code paths so USE_SVS=OFF builds

[plusky]

cmake/svs.cmake offers USE_SVS as a real option (cmake_dependent_option(USE_SVS "Build with SVS library support" ON "SVS_SUPPORTED" OFF)), but building with -DUSE_SVS=OFF does not compile.

With SVS off, HAVE_SVS is 0 and the ScalableVectorSearch sources are never fetched — but two translation units still reach for SVS unconditionally:

src/VecSim/algorithms/svs/svs.h:21:10: fatal error: svs/index/vamana/dynamic_index.h: No such file or directory
    (via tiered_factory.h -> svs_tiered.h -> svs.h)

src/VecSim/algorithms/svs/svs_utils.h:16:10: fatal error: svs/core/distance.h: No such file or directory
    (via vec_sim.cpp)

tiered_factory.h needs nothing from svs_tiered.h — none of its declarations mention an SVS type — so that include is simply guarded, the way tiered_factory.cpp and svs_factory.cpp already guard the same headers.

vec_sim.cpp does use VecSimSVSThreadPool, in two functions that are part of the public C API and so have to keep existing either way:

• VecSim_UpdateThreadPoolSize() sets the write mode and then resizes the shared SVS pool. With no SVS there is no pool, so setting the write mode is all the call has to do.
• VecSim_GetSharedMemory() reports the process-wide shared allocation. That is an SVS construct; without it nothing is held outside the individual indexes, which already report their own, so it returns 0.

No behaviour change when SVS is on — every added branch is under #if HAVE_SVS.

Why this matters outside your CI

I hit it packaging Redis 8.10.0 for openSUSE. Distribution builds have no network access at build time, and USE_SVS=ON wants eve, fmt, spdlog, tomlplusplus and robin-map cloned from git during cmake configure — plus the SVS_SHARED_LIB path, which downloads a pre-compiled shared library. -DUSE_SVS=OFF is exactly the escape hatch for that case, and these guards are all that stand between it and a working build. VectorSimilarity then uses its own HNSW and flat indexes, a perfectly good default for a distribution package.

Happy to add a USE_SVS=OFF configuration to CI in a follow-up so this stays fixed.

[CLAassistant]

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