Section Analysis (FusionHalfSectionViewCommand, INSPECT): cut the view by an origin plane, construction plane or planar face with an offset (drag arrow) and flip; the inside of cut bodies shows a flat cap colour; live while the dialog is open, Remove Section on the right-click menu; viewport shader validated in a test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/section.rs

@@ -0,0 +1,48 @@
+//! Section Analysis: a view-only cut through the model by a plane (what is on the plane's
+//! positive side is hidden). It changes no geometry and is not an undo step.
+
+use serde_json::{Value, json};
+use solvecraft_geom::{Plane, Vec3};
+
+use super::CommandSpec;
+use crate::params::{bad, bool_, expr, vec3};
+use crate::{Result, Session};
+
+pub static COMMANDS: &[CommandSpec] = &[CommandSpec::new("FusionHalfSectionViewCommand", "Section Analysis", section)
+    .at("SOLID", "INSPECT")
+    .icon("section")
+    .noundo()
+    .params("plane: XY|XZ|YZ | {origin, normal}; offset?: expr along the normal; flip?: bool; or clear: true")];
+
+fn section(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionHalfSectionViewCommand";
+    if bool_(p, "clear").unwrap_or(false) {
+        s.section = None;
+        s.revision += 1;
+        return Ok(json!({"section": null}));
+    }
+    let (origin, normal) = match p.get("plane") {
+        Some(Value::String(n)) => {
+            let pl = Plane::named(n).ok_or_else(|| bad(cmd, format!("unknown plane `{n}` (XY, XZ, YZ)")))?;
+            (pl.origin, pl.normal())
+        }
+        Some(o @ Value::Object(_)) => {
+            let origin = o.get("origin").and_then(vec3).unwrap_or(Vec3::ZERO);
+            let n = o.get("normal").and_then(vec3).and_then(|n| n.normalized()).ok_or_else(|| bad(cmd, "the plane needs a non-zero `normal`"))?;
+            (origin, n)
+        }
+        _ => return Err(bad(cmd, "`plane` must be XY, XZ, YZ or {origin, normal}")),
+    };
+    let offset = match expr(p, "offset") {
+        Some(e) => s.doc.eval(&e, solvecraft_doc::expr::Kind::Length).map_err(|e| bad(cmd, format!("offset: {e}")))?,
+        None => 0.0,
+    };
+    if !origin.is_finite() || !offset.is_finite() {
+        return Err(bad(cmd, "the plane must be finite"));
+    }
+    let normal = if bool_(p, "flip").unwrap_or(false) { -normal } else { normal };
+    let origin = origin + normal * offset * if bool_(p, "flip").unwrap_or(false) { -1.0 } else { 1.0 };
+    s.section = Some((origin, normal));
+    s.revision += 1;
+    Ok(json!({"section": {"origin": origin, "normal": normal}}))
+}