1D elements I: trusses (rod2, rod3), point masses, rotary inertia, springs and dashpots (#38)

* Model, fem and decks: point masses, rotary inertia, springs, dashpots and rod3 trusses

Model: PointMass and Spring definitions (spring | dashpot; to ground or
two-point; along a degree of freedom, a vector or the line of the ends) on a
set or reference point, resolved per node; ElementKind::Rod3. fem: rod2/rod3
truss kernel on Basis::Line2/Line3 declaring ElementDofs (translations).
Decks: Abaqus T3D3, MASS, ROTARYI, SPRING1/2/A, DASHPOT1/2/A; Nastran CONM1,
CELAS2, CDAMP2; LS-DYNA ELEMENT_MASS/INERTIA and ELEMENT_DISCRETE; what a
format cannot take is reported. Commands model.mass.* and model.spring.*.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* Implicit solver: trusses, point masses, rotary inertia and springs on the DofLayout

Trusses declare their translations like solids; point masses, inertia and
springs ask the layout for the kinds they need at their nodes (translations,
rotations), and reference points they use get six degrees of freedom. The
matrix pattern takes rows for those points; dashpots are ignored with a
warning. Existing results are bit-identical (hash-checked).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* Explicit solver: trusses, point masses, springs and dashpots on the node-major layout

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: fem.md on trusses and discrete elements in the layout; regenerate features.md

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: trusses in plane models and corotational explicit trusses

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/features.md

@@ -5,7 +5,7 @@ Generated from the capability catalogue (`crates/engine/src/catalogue.rs`) by
 Every entry is a toolbar button. **Ready** entries run their command; **coming soon** entries are
 shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 
-48 ready, 53 coming soon.
+50 ready, 51 coming soon.
 
 ## Geometry
 
@@ -106,8 +106,8 @@ shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 | Contact | Find Contacts — Detect contact pairs automatically | coming soon (M7) |  |
 | Contact | Rigid Wall — Rigid walls for explicit dynamics | coming soon (M10) |  |
 | Connectors | Bolt Load — Bolt pretension | coming soon (M7) |  |
-| Connectors | Spring — Springs and dashpots | coming soon (M7) |  |
-| Connectors | Point Mass — Point masses and inertia | coming soon (M7) |  |
+| Connectors | Spring — Spring or dashpot to the ground or between two points, along a direction or their line | ready | `model.spring.create`, `model.spring.edit`, `model.spring.delete` |
+| Connectors | Point Mass — Point mass and rotary inertia at nodes or a reference point | ready | `model.mass.create`, `model.mass.edit`, `model.mass.delete` |
 | Connectors | Connector — Connector elements between points | coming soon (M7) |  |
 | Coupled | FSI Interface — The fluid–structure interface | coming soon (M13) |  |