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/heat.rs

@@ -308,6 +308,8 @@ fn factor(a: &Csr, limits: &Limits, mesh: &HeatMesh, eqs: &Equations) -> Result<
         FactorError::TooLarge { entries, limit } => {
             Error::Limit(format!("the factorization needs {entries} entries; the limit is {limit} (refine less or raise the limit)"))
         }
+        FactorError::OutOfMemory { entries } => Error::Limit(format!("not enough memory for the factorization ({entries} entries)")),
+        FactorError::Internal(what) => Error::Internal(what),
         FactorError::NotPositiveDefinite { column } => {
             let label = eqs.node.get(column).and_then(|n| mesh.labels.get(*n)).copied().unwrap_or(0);
             Error::Singular(format!(