Model definition: sets by geometry name, materials, sections, couplings, BCs, loads, steps, check (#16)

* model: split nodes and elements into a mesh module (no behaviour change)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* model: sets by geometry name, materials and units, sections, couplings, BCs, loads, steps and the check

Sets bind to persistent geometry names through the mesh's geometry tags
(element, element face/edge and node classification) and resolve against
the current mesh, so they survive a remesh. Steps list their active loads
and boundary conditions explicitly. Model::check names every missing or
bad item; Model::resolve gives the writers' view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: model definition commands (sets, materials, sections, RPs, couplings, BCs, loads, steps, tags, check)

Each definition has create/edit/delete with its model JSON form as
parameters; model.step.activate and model.activity drive the explicit step
table; mesh.tag records where the mesh lies on the geometry; model.check
and model.set.resolve report against the current mesh. Hostile params run
against an empty and a fully defined session.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: model definition in architecture and roadmap

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* model, engine: review cleanups

Validated deserialization (a file replays through the checked methods),
interned geometry tags by index, set resolution with tag bitsets and sorted
vectors, one delete policy (a load or BC listed by a step is in use),
shared helpers (context, remove_existing, step_mut, opt/req/name/listed),
dead code removed, Resolved without a per-element section map.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* model: mesh-independent force distribution, tractions, stable reference point labels

Force and moment on a set default to a total, split by consistent nodal
weights over the set's faces (or equally over a node set); per_node is an
explicit opt-in. Traction is force per area. resolve() expands them to
nodal values for every writer and the solver. Reference points keep a
stored node label taken from the top of the label range.

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>

crates/engine/src/cmd/mod.rs

@@ -1,11 +1,14 @@
 //! The command registry. Each area has its own module; [`ALL`] lists every command.
 
+pub(crate) mod define;
 mod geometry;
 mod mesh;
 mod model;
+mod tags;
 
 use modelcraft_model::MAX_LABEL;
-use serde_json::Value;
+use serde::Deserialize;
+use serde_json::{Value, json};
 
 use crate::{CommandSpec, EngineError, Result};
 
@@ -22,6 +25,37 @@ pub static ALL: &[CommandSpec] = &[
     mesh::QUALITY,
     mesh::DELAUNAY_POINTS,
     mesh::SURFACE_CHECK,
+    tags::TAG,
+    tags::TAG_CLEAR,
+    model::UNITS,
+    model::CHECK,
+    model::SET_RESOLVE,
+    define::SET_CREATE,
+    define::SET_EDIT,
+    define::SET_DELETE,
+    define::MATERIAL_CREATE,
+    define::MATERIAL_EDIT,
+    define::MATERIAL_DELETE,
+    define::SECTION_CREATE,
+    define::SECTION_EDIT,
+    define::SECTION_DELETE,
+    define::REFERENCE_POINT_CREATE,
+    define::REFERENCE_POINT_EDIT,
+    define::REFERENCE_POINT_DELETE,
+    define::COUPLING_CREATE,
+    define::COUPLING_EDIT,
+    define::COUPLING_DELETE,
+    define::BOUNDARY_CONDITION_CREATE,
+    define::BOUNDARY_CONDITION_EDIT,
+    define::BOUNDARY_CONDITION_DELETE,
+    define::LOAD_CREATE,
+    define::LOAD_EDIT,
+    define::LOAD_DELETE,
+    define::STEP_CREATE,
+    define::STEP_EDIT,
+    define::STEP_DELETE,
+    define::STEP_ACTIVATE,
+    define::ACTIVITY,
 ];
 
 /// A required label parameter (1..=u32::MAX; the model checks its own range).
@@ -72,3 +106,25 @@ pub(crate) fn optional_number(p: &Value, key: &str, default: f64) -> Result<f64>
         Some(v) => v.as_f64().ok_or_else(|| EngineError::Params(format!("`{key}` must be a number"))),
     }
 }
+
+/// The `name` parameter.
+pub(crate) fn name(p: &Value) -> Result<&str> {
+    p.get("name").and_then(Value::as_str).ok_or_else(|| EngineError::Params("`name` must be a name".into()))
+}
+
+/// An optional parameter read as `T`'s JSON form; `None` when it is absent.
+pub(crate) fn opt<'a, T: Deserialize<'a>>(p: &'a Value, key: &str) -> Result<Option<T>> {
+    p.get(key).map(|v| T::deserialize(v).map_err(|e| EngineError::Params(format!("`{key}`: {e}")))).transpose()
+}
+
+/// A required parameter read as `T`'s JSON form.
+pub(crate) fn req<'a, T: Deserialize<'a>>(p: &'a Value, key: &str) -> Result<T> {
+    opt(p, key)?.ok_or_else(|| EngineError::Params(format!("`{key}` is required")))
+}
+
+/// Entries listed in a report: the first ones, and how many there are.
+pub(crate) const LISTED: usize = 100;
+
+pub(crate) fn listed<T: serde::Serialize>(v: &[T]) -> Value {
+    json!({"count": v.len(), "labels": v.get(..LISTED).unwrap_or(v)})
+}