fix(tess): clamp isoline counts before they reach the kernel

Bug:
$ISOLINES (header i16) and Surface.u/v_isolines (file-controlled i16)
were only ever floored at 0, never capped, and were handed to the
tessellation kernel as the per-face isolate count - which draws
0..count isolines per face with no ceiling of its own. SETVAR ISOLINES
32767 (or a DWG carrying that header, or a Surface with u_isolines =
32767) turned every rebuild and every per-cursor preview of
PRESSPULL/REVOLVE/SWEEP/LOFT into a multi-million-vertex computation:
measured 4,259,966 edge vertices for one 12-unit cylinder preview.

Reproduce:
1. SETVAR ISOLINES 32767 (or open a DWG whose $ISOLINES is large, or
   edit a Surface whose u_isolines is 32767).
2. Draw a circle and start REVOLVE (or PRESSPULL/SWEEP/LOFT), or shade
   any 3D solid.
3. Before: the app allocates without bound - abort on memory failure or
   a permanent freeze; previews recompute on every mouse move.
4. After: previews stay interactive; a hostile header renders exactly
   like the largest value the Options slider can request.

Fix:
Cap each value at the ceiling this product already agrees on, where the
type changes from i16 to the kernel count: header $ISOLINES <= 64 (the
Options slider's own "Isolines per surface" range), Surface
u/v_isolines <= 200 (the plugin editor's writable range). One helper
(clamp_header_isolines) replaces all 11 copies of .max(0) as usize
across the entity/scene/control/display feed sites,
surface_isoline_counts clamps both axes, and SETVAR ISOLINES stores
x.clamp(0, 64) so the echo, slider and re-saved files agree with what
renders; file-sourced headers keep their raw stored value and are
clamped at consumption. Clamping is density-only: geometry, bounds and
saves are untouched. Three regression tests cover hostile surface
counts, the SETVAR entry, and a file-sourced header measured through
display tessellation (hostile renders like the slider ceiling, negative
renders like zero).

src/scene/entity.rs

@@ -319,7 +319,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         let mesh_seed = if matches!(
             &entity,
             EntityType::Solid3D(_)
@@ -603,7 +603,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         let mesh_seed = if matches!(
             &entity,
             EntityType::Solid3D(_)
@@ -832,7 +832,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         use crate::par::prelude::*;
         let built: Vec<(Handle, MeshLodSet, bool)> = entries
             .into_par_iter()
@@ -2578,7 +2578,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         // Top-level solids: offset into the render frame, drawn flat.
         // Block-definition solids: keep block-local coords for per-INSERT
         // instancing (no offset applied here).