Add deps/build.jl to download F3D before precompilation

Downloads.download during precompilation leaves curl timers alive,
causing "waiting for IO to finish" warnings and potentially corrupt
precompiled caches (segfaults in CI). By downloading in build.jl
(which runs before precompilation via Pkg.build), ensure_f3d() finds
the cached library during precompilation and never calls Downloads.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: joa-quim

deps/build.jl

@@ -0,0 +1,10 @@
+include("../src/F3D.jl")
+using .F3D: ensure_f3d
+
+try
+    lib_dir = ensure_f3d()
+    @info "F3D build complete. Library directory: $lib_dir"
+catch e
+    @error "F3D build failed" exception = (e, catch_backtrace())
+    rethrow()
+end