Add Warp flow, a video effect that sets Warp's noise field drifting

Warp's noise field moves to effects/noise_field.hpp so both share it; Warp
renders exactly as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core_tests.cpp

@@ -2107,6 +2107,7 @@ int main() {
         };
         const std::vector<Moving> moving = {
             {"tunnel", {{"tunnel_drift", 6L}, {"tunnel_fade", 1.0}}, {}, {{"tunnel_fade", 0.0}}},
+            {"flow", {{"flow_mode", std::string("swirl")}}, {}, {{"flow_strength", 0L}}},
         };
         for (const auto& [name, lively, smooth, off] : moving) {
             const auto effect = effectByName(name);