Rename the project to FieldCraft

Crates fieldcraft-*, binaries fieldcraft and fieldcraft-cli, the .fieldcraft file (format tag
fieldcraft/1, manifest fieldcraft.json), FIELDCRAFT_* environment variables, the MCP server
name, egui widget ids (fc_*), the app icon document FieldCraft.icon and its generated family,
docs and licences. No compatibility shim: no files of the earlier name exist.

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

crates/solver/src/eigen.rs

@@ -286,7 +286,7 @@ pub fn lanczos(op: &impl Operator, shift: f64, wanted: Wanted, block: usize, max
         // W = Op Q_j.
         let mut w: Vec<Vec<f64>> = Vec::with_capacity(b);
         // The block's solves are independent: one task each.
-        for y in modelcraft_parallel::map(mcur.len(), |i| {
+        for y in fieldcraft_parallel::map(mcur.len(), |i| {
             let mut y = mcur.get(i).cloned().unwrap_or_default();
             op.solve(&mut y).map(|()| y)
         }) {