Abaqus input writer and deck.write (M3) (#22)

* decks: Abaqus input writer with golden decks, read-back, Jacobian and unsupported tests

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

* engine: deck.write command (format dispatch, atomic file write), docs

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

* decks: format table, one resolve per write, direct buffer writes; engine: shared path parameter

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/engine/src/cmd/deck.rs

@@ -0,0 +1,65 @@
+//! Solver decks: the model written in a solver's input format.
+
+use std::path::Path;
+
+use serde_json::{Value, json};
+
+use super::req;
+use crate::{CommandKind, CommandSpec, EngineError, Result, Session};
+
+pub const WRITE: CommandSpec = CommandSpec {
+    id: "deck.write",
+    label: "Write Solver Deck",
+    params: r#"{"format": "abaqus", "path": "part.inp"} — writes the model as a solver deck (formats: modelcraft_decks::FORMATS); fails naming every error of the model check and everything the format cannot express"#,
+    kind: CommandKind::Session,
+    run: write,
+};
+
+fn write(s: &mut Session, p: &Value) -> Result<Value> {
+    let name: &str = req(p, "format")?;
+    let format = modelcraft_decks::format(name).ok_or_else(|| {
+        let shown: String = name.chars().take(32).collect();
+        let known: Vec<&str> = modelcraft_decks::FORMATS.iter().map(|f| f.name).collect();
+        EngineError::Params(format!("unknown deck format `{shown}`; known: {}", known.join(", ")))
+    })?;
+    let path = super::path_param(p, "path")?;
+    let deck = format.write(&s.model)?;
+    modelcraft_io::fs::write_atomic(Path::new(path), deck.text.as_bytes(), false)?;
+    Ok(json!({"format": format.name, "path": path, "bytes": deck.text.len(), "warnings": deck.warnings}))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn writes_the_defined_model_and_refuses_what_it_cannot() {
+        let dir = std::env::temp_dir().join(format!("modelcraft-deck-write-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join("bar.inp");
+        let path_str = path.to_string_lossy().into_owned();
+
+        let mut s = Session::default();
+        crate::cmd::define::tests::defined_bar(&mut s);
+        let journal = s.journal.len();
+        let out = s.execute("deck.write", &json!({"format": "abaqus", "path": path_str})).unwrap();
+        let written = std::fs::read_to_string(&path).unwrap();
+        assert_eq!(written, modelcraft_decks::abaqus::write(&s.model).unwrap());
+        assert_eq!(out["bytes"], written.len());
+        assert_eq!(out["warnings"].as_array().map(Vec::len), Some(1), "`press` is in no step: {out}");
+        assert!(written.contains("*ELEMENT, TYPE=C3D8I"), "{written}");
+        assert_eq!(s.journal.len(), journal, "writing a deck does not change the model");
+        assert!(std::fs::read_dir(&dir).unwrap().count() == 1, "no partial file is left behind");
+
+        // Unknown formats, missing parameters and models that are not ready fail without writing.
+        let other = dir.join("other.inp").to_string_lossy().into_owned();
+        assert!(s.execute("deck.write", &json!({"format": "nope", "path": other})).unwrap_err().to_string().contains("known: abaqus"));
+        assert!(s.execute("deck.write", &json!({"path": other})).is_err());
+        assert!(s.execute("deck.write", &json!({"format": "abaqus"})).is_err());
+        let mut empty = Session::default();
+        assert!(matches!(empty.execute("deck.write", &json!({"format": "abaqus", "path": other})), Err(EngineError::Deck(_))));
+        assert!(!dir.join("other.inp").exists());
+        assert!(matches!(s.execute("deck.write", &json!({"format": "abaqus", "path": dir.to_string_lossy()})), Err(EngineError::File(_))));
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+}

crates/engine/src/cmd/mod.rs

@@ -1,5 +1,6 @@
 //! The command registry. Each area has its own module; [`ALL`] lists every command.
 
+pub(crate) mod deck;
 pub(crate) mod define;
 mod edit;
 pub(crate) mod file;
@@ -66,6 +67,7 @@ pub static ALL: &[CommandSpec] = &[
     file::RECOVERY_DISCARD,
     edit::UNDO,
     edit::REDO,
+    deck::WRITE,
 ];
 
 /// A required label parameter (1..=u32::MAX; the model checks its own range).