Log which body/step fails inside tessellate_sat() instead of a silent None

Every early-out here (a bad-body abort, or zero triangles produced) used
to vanish with no trace anywhere, including upstream's own UI - the exact
reason the open '3D solids don't survive save/reopen' bug has resisted
diagnosis so far. Now logs the body index, which step failed, and (for the
zero-bodies case) how many surfaces/curves/pointers lift() couldn't
resolve, so the next real occurrence is diagnosable from the browser
console instead of another silent None. No behavior change - purely
diagnostic.

src/scene/convert/acis_kernel.rs

@@ -27,13 +27,43 @@ pub fn tessellate_sat(
     isolines: usize,
 ) -> Option<MeshLodSet> {
     let (bodies, loss) = lift(document);
+    let body_count = bodies.len();
     if bodies.is_empty() {
+        log::warn!(
+            "tessellate_sat({name}): lift() returned zero bodies - {} unsupported surface(s), {} unsupported curve(s), {} broken/malformed pointer(s) - nothing to draw",
+            loss.surfaces.len(),
+            loss.curves.len(),
+            loss.broken.len()
+        );
         return None;
     }
-    let mut placed_bodies = Vec::with_capacity(bodies.len());
+    // Each of the three early-outs below used to be a bare `?`, which - since
-    for body in bodies {
+    // this is a plain `for` loop, not a closure - aborted the *entire*
-        let source = body.provenance.source()?;
+    // document (discarding every other body's perfectly good geometry, not
-        let transform = body_transform(document, source.index() as usize).ok()?;
+    // just the one that failed) with zero trace anywhere, including
+    // upstream's own UI. That's the exact "reopen shows nothing, Zoom
+    // Extents finds nothing" symptom this function has been blamed for.
+    // Logging *which* body and *which* step failed turns the next real
+    // occurrence into something diagnosable from a browser console instead
+    // of another silent None (CMMS incident, 2026-08-27).
+    let mut placed_bodies = Vec::with_capacity(body_count);
+    for (index, body) in bodies.into_iter().enumerate() {
+        let Some(source) = body.provenance.source() else {
+            log::warn!(
+                "tessellate_sat({name}): body {index}/{body_count} has no provenance source - aborting the whole document instead of just this body"
+            );
+            return None;
+        };
+        let transform = match body_transform(document, source.index() as usize) {
+            Ok(t) => t,
+            Err(err) => {
+                log::warn!(
+                    "tessellate_sat({name}): body {index}/{body_count} (source record {}) has no resolvable transform: {err:?} - aborting the whole document",
+                    source.index()
+                );
+                return None;
+            }
+        };
         let placement_scale = transform.map_or(1.0, |(_, _, scale)| scale.abs());
         let placed = if let Some((matrix, translation, scale)) = transform {
             let placement = brep::Placement {
@@ -42,7 +72,13 @@ pub fn tessellate_sat(
                 z_axis: [scale * matrix[6], scale * matrix[7], scale * matrix[8]],
                 origin: translation,
             };
-            brep::transform(&body, &placement)?
+            let Some(placed) = brep::transform(&body, &placement) else {
+                log::warn!(
+                    "tessellate_sat({name}): body {index}/{body_count} failed brep::transform() - aborting the whole document"
+                );
+                return None;
+            };
+            placed
         } else {
             body
         };
@@ -160,6 +196,9 @@ pub fn tessellate_sat(
         }
     }
     if indices.is_empty() {
+        log::warn!(
+            "tessellate_sat({name}): {body_count} placed bodies produced zero triangles ({undrawn} face(s) undrawn) - every face was either missing or unrepresentable in its surface's own parameters"
+        );
         return None;
     }