PERF-2 P2: the red-black colour maps, the residual and the V-cycle's per-level maps on rayon threads (MultigridParameters::threads; RTX_THREADS in the harness; rtx_cfd::configure_threads) — each colour's values computed from the unchanged other colour into a scratch and written back, the L1 sum in the serial order: bit-identical to the serial red-black (pin: 4 right-hand sides at 4 threads); the coarsest level stays serial; no effect on the lexicographic regime

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YJPeT6WA2e7YvAnS875AHL

crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs

@@ -329,6 +329,11 @@ impl OversetPisoSolver {
         })
     }
 
+    /// PERF-2 P2: threads for the background multigrid's red-black maps.
+    pub fn set_poisson_threads(&mut self, threads: usize) {
+        self.background.set_poisson_threads(threads);
+    }
+
     /// The step timers, when profiling (`RTX_PROFILE`).
     pub fn timers(&self) -> Option<&StepTimers> {
         self.timers.as_deref()