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>

ATTRIBUTION.md

@@ -14,6 +14,7 @@ is committed and listed here with its exports.
 
 | File | Author | Source | Licence |
 |---|---|---|---|
+| crates/decks/tests/golden/nastran/*.bdf | ModelCraft contributors | Written by our Nastran writer from our own test models (crates/decks/tests/nastran.rs) | MIT OR Apache-2.0 |
 
 ## Adapted code