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>

docs/architecture.md

@@ -22,7 +22,7 @@ crates in strictly lower layers.
 | L1 | `modelcraft-geometry` | The only door to SolidCraft: opens `.solidcraft` designs (rebuilt from their timeline, bodies with persistent face names), B-rep bodies, measures, tessellation into a face-tagged `SurfaceMesh`; face-tagged 3MF files read into the same type without the kernel (docs/formats/tagged-mesh.md). No SolidCraft type in its API. | started |
 | L1 | `modelcraft-mesh` | Surface preparation (weld, watertightness, feature edges), robust predicates, Delaunay tetrahedralization and refinement, Voronoi-based optimization, hex meshing, quality metrics. | started |
 | L1 | `modelcraft-solver` | Element formulations, assembly, sparse linear static and modal solution. | planned |
-| L1 | `modelcraft-decks` | Abaqus input, Nastran bulk data and LS-DYNA keyword readers and writers, to and from `model`. | planned |
+| L1 | `modelcraft-decks` | Abaqus input, Nastran bulk data and LS-DYNA keyword readers and writers, to and from `model`. Text out, no file system; `deck.write` saves it. | started |
 | L1 | `modelcraft-render` | Display data without a GPU dependency: the visible faces, element edges and nodes of a model, the orbit camera, a CPU rasterizer for headless snapshots; contour maps later. | started |
 | L2 | `modelcraft-doc` | The document: geometry, mesh, model, results, history. | planned |
 | L3 | `modelcraft-io` | `.modelcraft` files (docs/formats/modelcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery. Bytes in, bytes out; the file system only in `fs` and `recovery`. Result files later. | started |

docs/parity.md

@@ -56,7 +56,7 @@ products are run, no documentation text is copied). Status: **done**, **started*
 | Interaction | Tie between non-matching meshes | M7 | | planned |
 | Interaction | Contact (general and pairs), friction, automatic pair detection | M7 | | planned |
 | Interaction | Bolt pretension, springs, point masses, connectors | M7 | | planned |
-| Decks | Abaqus input writer | M3 | | planned |
+| Decks | Abaqus input writer (`deck.write`, flat deck; reports what Abaqus cannot express) | M3 | | done |
 | Decks | Nastran bulk data writer (SOL 101, 103) | M3 | | planned |
 | Decks | LS-DYNA keyword writer (implicit; explicit in M8) | M3 | | planned |
 | Decks | Readers keeping unknown keywords | M9 | | planned |
@@ -76,3 +76,8 @@ products are run, no documentation text is copied). Status: **done**, **started*
   as degenerate quads. Every mapping is tested by the sign of the element Jacobian after writing.
 - Each format's field widths, continuation rules, number forms and label limits are respected by
   the writer and checked by golden-file tests on our own examples.
+- Abaqus: the model's node order and face numbers are Abaqus's, so connectivity passes through;
+  element types come from the element kind, the section and its formulation (table in
+  `crates/decks/src/abaqus.rs`). The step table is bridged to Abaqus's propagation: zero-valued
+  boundary conditions active in every step go to the initial state, and steps restate their
+  boundary conditions and loads with `OP=NEW` when the active set changes.