Nastran bulk data writer (SOL 101/103) and deck.write steps (M3) (#24)

* Nastran bulk data writer: fixed-format entries, exact large-field reals, SOL 101/103 decks

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

* Nastran writer tests: golden decks, a re-reader of our output, node order against Nastran's conventions

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

* docs: Nastran writer in the roadmap, parity table and deck notes

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

* Nastran writer: simplify planning (subcases, one property table), stream cards into the deck, write through the VFS

ElementKind::corner_count/is_quadratic and Dofs::bits/from_bits join the model.

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

* Nastran joins the shared deck formats; deck.write chooses steps for every format

nastran::write_resolved is the FORMATS row; Format::write_steps and with_steps write a subset of
steps (a Nastran run is static or frequency steps); problems go through the shared unsupported().

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/model/src/bcs.rs

@@ -32,6 +32,16 @@ impl Dofs {
         self.0 == 0
     }
 
+    /// The degrees of freedom as bits: bit `d - 1` for degree of freedom `d`.
+    pub fn bits(self) -> u8 {
+        self.0
+    }
+
+    /// The degrees of freedom of bits (bits above the sixth are ignored).
+    pub fn from_bits(bits: u8) -> Dofs {
+        Dofs(bits & 0b111_111)
+    }
+
     /// The degrees of freedom in order.
     pub fn list(self) -> Vec<u8> {
         (1..=6).filter(|d| self.contains(*d)).collect()

crates/model/src/mesh.rs

@@ -76,6 +76,22 @@ impl ElementKind {
         }
     }
 
+    /// Corner nodes: the first ones in the node list (the rest are mid-side nodes).
+    pub fn corner_count(self) -> usize {
+        match self {
+            ElementKind::Rod2 | ElementKind::Beam2 => 2,
+            ElementKind::Tri3 | ElementKind::Tri6 => 3,
+            ElementKind::Quad4 | ElementKind::Quad8 | ElementKind::Tet4 | ElementKind::Tet10 => 4,
+            ElementKind::Wedge6 => 6,
+            ElementKind::Hex8 | ElementKind::Hex20 => 8,
+        }
+    }
+
+    /// Whether the element has mid-side nodes.
+    pub fn is_quadratic(self) -> bool {
+        self.node_count() > self.corner_count()
+    }
+
     /// Topological dimension: 1 for line elements, 2 for shells, 3 for solids.
     pub fn dimension(self) -> u8 {
         match self {
@@ -387,6 +403,10 @@ mod tests {
         assert_eq!(ElementKind::Tet4.faces().len(), 4);
         assert_eq!(ElementKind::Wedge6.faces().len(), 5);
         assert_eq!(ElementKind::Hex20.edges().len(), 12);
+        for kind in ElementKind::ALL {
+            let mid_sides = kind.edges().iter().filter(|e| e.len() == 3).count();
+            assert_eq!(kind.corner_count() + mid_sides, kind.node_count(), "{kind:?}");
+        }
     }
 
     #[test]