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/constraints.rs

@@ -51,9 +51,9 @@
 
 use std::collections::BTreeMap;
 
-use modelcraft_fem::dense::inverse3;
-use modelcraft_fem::{DofKind, DofSet};
-use modelcraft_model::{CouplingKind, Model, Resolved};
+use fieldcraft_fem::dense::inverse3;
+use fieldcraft_fem::{DofKind, DofSet};
+use fieldcraft_model::{CouplingKind, Model, Resolved};
 
 use crate::sparse::Csr;
 use crate::system::Mesh;