Deterministic CPU multithreading: supernodal Cholesky, parallel elements, LBM blocks (#50)

* parallel: deterministic thread pool helpers for the solvers (fixed partitions, ordered results)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* solver: supernodal left-looking Cholesky, multithreaded with a fixed operation order

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* solver: parallel element matrices, stresses, Lanczos block solves and products (sums in the sequential order)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* explicit: element updates in parallel, nodal forces added in element order (bit-identical to the sequential loop)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* lbm: collision and streaming by fixed cell blocks in parallel, wall forces in link order (bit-identical)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: threads setting for the solve commands (session default from MODELCRAFT_THREADS or the cores); bit-identity test on 1, 2 and 7 threads

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: rayon threads in ADR 0001, the parallel crate, supernodal Cholesky

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/solver/src/eigen.rs

@@ -184,7 +184,7 @@ pub struct Wanted {
 }
 
 /// The operators the Lanczos iteration needs.
-pub trait Operator {
+pub trait Operator: Sync {
     fn n(&self) -> usize;
     /// y = M x.
     fn mass(&self, x: &[f64], y: &mut [f64]);
@@ -285,10 +285,12 @@ pub fn lanczos(op: &impl Operator, shift: f64, wanted: Wanted, block: usize, max
         let j0 = q.len();
         // W = Op Q_j.
         let mut w: Vec<Vec<f64>> = Vec::with_capacity(b);
-        for mx in &mcur {
-            let mut y = mx.clone();
-            op.solve(&mut y)?;
-            w.push(y);
+        // The block's solves are independent: one task each.
+        for y in modelcraft_parallel::map(mcur.len(), |i| {
+            let mut y = mcur.get(i).cloned().unwrap_or_default();
+            op.solve(&mut y).map(|()| y)
+        }) {
+            w.push(y?);
         }
         // A_j = Q_jᵀ M W (symmetrized), W −= Q_j A_j (the earlier blocks go in the full
         // reorthogonalization below).