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 <[email protected]>
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 <[email protected]>
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 <[email protected]>
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 <[email protected]>
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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/parity.md

@@ -57,7 +57,7 @@ products are run, no documentation text is copied). Status: **done**, **started*
 | Interaction | Contact (general and pairs), friction, automatic pair detection | M7 | | planned |
 | Interaction | Bolt pretension, springs, point masses, connectors | M7 | | 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 | Nastran bulk data writer (`deck.write`, SOL 101 or 103 per deck; reports what Nastran cannot express) | M3 | | done |
 | Decks | LS-DYNA keyword writer (implicit; explicit in M8) | M3 | | planned |
 | Decks | Readers keeping unknown keywords | M9 | | planned |
 | Solve | Own linear static and modal solver, verified against analytic solutions | M4 | yes | planned |
@@ -81,3 +81,10 @@ products are run, no documentation text is copied). Status: **done**, **started*
   `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.
+- Nastran (`modelcraft_decks::nastran`): entries with a real use large (16-column) fields, the rest
+  small fields; a real is exact when its shortest round-trip form fits 16 columns, else rounded to
+  at least ten significant digits. Linear solids map full integration to `PSOLID` `TWO`/`THREE` +
+  `FULL`, hex20 reduced to `TWO` + `FULL`, hex8 incompatible modes to `BUBBLE`; one-point reduced
+  linear solids, hybrid solids, reduced-integration or one-point shells, body forces, gravity on part
+  of the model, orthotropic beams and rods, and orthotropic shells off planes of constant Z are
+  reported as unsupported. Static and frequency steps need separate decks (`deck.write` `steps`).