Fix Windows build and test suite compatibility

[x-Spartacus]

Replace shell-specific build asset copy with a Bun script so bun run build works on Windows.
Normalize mocked config paths in config-resolution.test.ts so config resolution tests pass on Windows.
Add retrying temp-dir cleanup for Windows SQLite file-lock release.
Close managed/direct SQLite handles in vector tests before deleting temp dirs.
Skip USearchBackend-specific tests when the native usearch package cannot load in the current runtime.

Tested on Windows:

npx bun run build
npx tsc --noEmit
npx prettier --check package.json scripts/copy-web-assets.mjs tests/config-resolution.test.ts tests/helpers/temp-dir.mjs tests/profile-write.test.ts tests/vector-search-backend-integration.test.ts tests/vector-backends/exact-scan-backend.test.ts tests/vector-backends/migration-fallback.test.ts tests/vector-backends/usearch-backend.test.ts
npx bun test

test result:

150 pass
4 skipped
0 fail

[tickernelz]

Merged ✅

Verification on merged main:

• Local merge-test clean (no conflicts)
• bun test → 154 pass, 0 fail
• bun run typecheck → pass
• bun run build → pass

What this brings in:

• Cross-platform build (bun scripts/copy-web-assets.mjs replacing shell-specific cp -r)
• Windows-safe temp-dir cleanup with retry on EBUSY/ENOTEMPTY/EPERM
• Proper SQLite handle close() before temp-dir deletion in vector tests
• usearch tests now skip when the native package can't load
• CI green on Ubuntu / macOS / Windows (Embedding Backend Verification)