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/model/src/lib.rs

@@ -314,7 +314,7 @@ impl Model {
     }
 
     /// A copy with the unit system and every definition but no mesh: no nodes, elements or
-    /// geometry tags. A `.modelcraft` file stores this part as text and the mesh in binary.
+    /// geometry tags. A `.fieldcraft` file stores this part as text and the mesh in binary.
     pub fn without_mesh(&self) -> Model {
         Model {
             nodes: BTreeMap::new(),

crates/model/src/mesh.rs

@@ -105,7 +105,7 @@ impl ElementKind {
         }
     }
 
-    /// The name used in commands and `.modelcraft` files (`hex8`, `tet10`, …).
+    /// The name used in commands and `.fieldcraft` files (`hex8`, `tet10`, …).
     pub fn name(self) -> &'static str {
         match self {
             ElementKind::Rod2 => "rod2",