PERF-2 P1.1: the multigrid-PCG's prepared operator (hierarchy, f64 fine level, active cells, components) cached on the embedded solver and reused while the operator is bit-identical (an exact key over the coefficient bit patterns, the mask and the hierarchy parameters); solve_multigrid_pcg_cached gives the uncached solve's answer bit for bit (pin poisson_cache_exact.rs: five right-hand sides on one operator, a one-coefficient miss, both precisions); the CG driver split into Prepared::build + run_pcg

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

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded/projection.rs

@@ -7,7 +7,7 @@ use super::EmbeddedPisoSolver;
 use crate::CfdResult;
 use crate::solvers::incompressible::ale::SideBoundary;
 use crate::solvers::incompressible::poisson::{
-    MultigridParameters, PoissonProblem, PoissonSolverKind, solve_multigrid_pcg,
+    MultigridParameters, PoissonProblem, PoissonSolverKind, solve_multigrid_pcg_cached,
 };
 use crate::solvers::incompressible::{EmbeddedMask, FlowField};
 
@@ -294,7 +294,7 @@ impl EmbeddedPisoSolver {
             }
             let anchor_cell =
                 (!any_outlet && !self.has_fringe()).then_some(anchor.0 * nx + anchor.1);
-            let solution = solve_multigrid_pcg(
+            let solution = solve_multigrid_pcg_cached(
                 &problem,
                 &mut p_prime,
                 &MultigridParameters {
@@ -303,6 +303,7 @@ impl EmbeddedPisoSolver {
                 },
                 inner_stop,
                 anchor_cell,
+                &mut self.pcg_cache.borrow_mut(),
             );
             let (s0, i0, c0, k0) = self.poisson_profile.get();
             self.poisson_profile.set((