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/architecture.md

@@ -138,8 +138,13 @@ module docs; ADR 0001 records why the linear algebra is our own.
    quad4, quad8), chosen by the section kind, and the model then has two degrees of freedom per
    node: the 2D elements declare `ElementDofs::new(DofSet::PLANE)` (Ux, Uy) to the `DofLayout`. A 2D model
    lies in the XY plane (axisymmetric: x the radius, Y the axis; nodes on the axis are held
-   radially), is loaded on element edges, and cannot mix with solids or couplings yet. Every reference point a coupling
-   ties to the mesh gets six degrees of freedom (translations, rotations) after the mesh's. Shells
+   radially), is loaded on element edges, and cannot mix with solids or couplings yet. Rod
+   sections' rod2 and rod3 elements are trusses (`modelcraft_fem::truss`). Point masses, rotary
+   inertia and springs (`Resolved::masses`, `springs`, expanded per node by the model) assemble on
+   the unknowns their nodes are given (`discrete::needs`, `discrete::dof_of`); dashpots are
+   ignored with a warning (no velocities in static and frequency steps). Every reference point a
+   coupling or a discrete element uses gets six degrees of freedom (translations, rotations) after
+   the mesh's. Shells
    and beams return "not supported yet". The step's boundary conditions prescribe translations
    (rotations do not exist on solid nodes) and a coupled point's six directions; its loads become
    a load vector: forces and tractions from `Resolved::nodal_loads`, forces and moments on coupled
@@ -181,7 +186,10 @@ and rigid-body modes.
 (kinematic and distributing), clamping through a coupled point equals clamping (static and
 modes), tension through a distributing coupling against the equivalent pressure away from the
 end, pure bending by a moment through a distributing coupling, torsion of a tube by a moment on
-a kinematically coupled point (twist and shear stress).
+a kinematically coupled point (twist and shear stress). `crates/solver/tests/one_d.rs`: member
+forces of determinate trusses (a plane Pratt truss, a space tripod) against the method of joints,
+the three-bar truss, a rod3 constant-stress patch and a bar hanging under its weight, spring–mass,
+skew springs, the two-mass chain, a bar with a tip mass and the torsional pendulum.
 
 ## Explicit solver
 
@@ -192,9 +200,11 @@ number their degrees of freedom with `modelcraft_fem::DofLayout` (docs/fem.md).
 
 1. **System.** `Model::resolve()`; hex8 sections with reduced integration become one-point
    uniform-strain hexahedra with Flanagan–Belytschko hourglass control, tet4 sections Bonet–Burton
-   average nodal pressure tetrahedra (`modelcraft_fem::uniform` holds their kinematics). Other
-   element kinds and formulations, couplings, reference points and moments return "not supported
-   yet". Masses are row-sum lumped. A zero-valued boundary condition holds a degree of freedom; a
+   average nodal pressure tetrahedra (`modelcraft_fem::uniform` holds their kinematics), rod2 and
+   rod3 trusses (`discrete`). Point masses add to the lumped mass; springs and dashpots (on the
+   previous half increment's velocity) add internal and viscous forces and their own stable
+   increment. Other element kinds and formulations, couplings, reference points, rotary inertia,
+   rotational springs and moments return "not supported yet". Masses are row-sum lumped. A zero-valued boundary condition holds a degree of freedom; a
    displacement value is reached at constant velocity over the step. Loads act in full from the
    start. The step lists initial velocities and fixed frictionless rigid planes by set.
 2. **Time step.** Each element's highest frequency is bounded in closed form (exact for a cube);
@@ -227,7 +237,9 @@ only): stress waves in hex8 and tet4 bars (front at c·t, σ = ρ c v, work inpu
 rigid wall (contact time 2L/c, rebound speed, plateau force ρ c v₀ A), the energy balance,
 bit-for-bit repeatability; Dienes' simple shear for both rates, a stressed element turned rigidly
 by 90°, the J2 uniaxial curve and unloading at large strain, damage failure at the fracture
-strain, and the Taylor bar against Taylor's 1948 estimate; in the engine (`solve.rs`), a
+strain, and the Taylor bar against Taylor's 1948 estimate; `crates/explicit/tests/one_d.rs`:
+spring–mass period, damped decay rate and period, a dashpot alone, two masses on a line spring,
+waves in rod2 and rod3 bars; in the engine (`solve.rs`), a
 cantilever's free-vibration period against `solve.frequency`.
 
 ## Lattice Boltzmann solver