[MayCXC]

dist/vec0.so exports every non-static function in sqlite-vec.c, and sqlite3_api with them: on Linux arm64, nm -D --defined-only dist/vec0.so lists 111 symbols, among them array_append, bitmap_new, the diskann_* helpers and vec0Filter_fullscan. SQLite opens an extension with RTLD_GLOBAL, so the calls inside one library can bind to another library's exported function of the same name. With two builds of sqlite-vec in one process, the second build runs the first build's code:

import sqlite3

# a: make loadable; b: the same source built with -DSQLITE_VEC_ENABLE_RESCORE=0
for path in ["a/vec0", "b/vec0"]:
    db = sqlite3.connect(":memory:")
    db.enable_load_extension(True)
    db.load_extension(path)
    db.execute(
        "create virtual table v using vec0("
        "embedding float[8] indexed by rescore(quantizer=bit))"
    )

Loaded alone, b fails with vec0 constructor error: could not parse vector column 'embedding float[8] indexed by rescore(quantizer=bit)'. Loaded after a, the process crashes with SIGSEGV on that statement.

This PR compiles with -fvisibility=hidden, as vlasky/sqlite-vec@e8f156b does for vlasky's fork (it says it was inspired by #149, which its author closed without a review) and as photostructure/sqlite-vec's Makefile does, which has that commit in its history. SQLITE_VEC_API gets default visibility so sqlite3_vec_init stays exported; nm -D then lists it alone, and with this PR the reproduction fails on b the way b fails when loaded alone. One difference from vlasky's header: the SQLITE_VEC_STATIC branch uses __attribute__((visibility("default"))) only with GCC and Clang outside Windows, so MSVC can still compile a SQLITE_VEC_STATIC build and MinGW does not warn about an ignored attribute. A Windows DLL already exports only its __declspec(dllexport) functions, so Windows builds are unchanged.

site/compiling.md's manual Linux and macOS commands pass the flag too, with a sentence on why; the documented Linux command, built from this branch, exports sqlite3_vec_init alone and loads.

Test. test_exports_only_entry_point opens dist/vec0 with ctypes and checks that sqlite3_vec_init resolves and sqlite3_api, bitmap_new and vec0Filter_fullscan do not. On main it fails with AssertionError: sqlite3_api. make test-loadable passes (240 passed, 66 skipped), and the build adds no warnings to main's 12.

This merges cleanly with #293, #304 and #305, which also touch the Makefile or the tests.