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/app/commands/styleprops.rs

@@ -2223,6 +2223,14 @@ impl OpenCADStudio {
                                 Some(v) => v
                                     .parse::<i16>()
                                     .map(|x| {
+                                        // Stored values stay inside the range
+                                        // the Options slider can ask for; the
+                                        // consumption sites clamp again for
+                                        // file-sourced headers.
+                                        let x = x.clamp(
+                                            0,
+                                            crate::entities::solid3d::MAX_HEADER_ISOLINES,
+                                        );
                                         h.isolines = x;
                                         (format!("ISOLINES = {x}"), true)
                                     })
@@ -3416,6 +3424,27 @@ mod tests {
         app
     }
 
+    /// `SETVAR ISOLINES` stored any parseable `i16` verbatim. The header
+    /// value is the per-face isolate count handed to the tessellation
+    /// kernel (which loops `0..count`), so `ISOLINES 32767` made every
+    /// rebuild and per-cursor preview allocate without bound. The stored
+    /// value must stay inside the range the app's own UI can ask for
+    /// (the Options slider is 0..=64).
+    #[test]
+    fn setvar_isolines_stays_within_the_renderable_range() {
+        let mut app = fresh_app();
+        let i = app.active_tab;
+
+        let _ = app.run_command_line("SETVAR ISOLINES 32767");
+        assert_eq!(app.tabs[i].scene.document.header.isolines, 64);
+
+        let _ = app.run_command_line("SETVAR ISOLINES 8");
+        assert_eq!(app.tabs[i].scene.document.header.isolines, 8);
+
+        let _ = app.run_command_line("SETVAR ISOLINES -3");
+        assert_eq!(app.tabs[i].scene.document.header.isolines, 0);
+    }
+
     #[test]
     fn selection_commands_gather_instead_of_using_the_whole_drawing() {
         for command in ["SETBYLAYER", "OVERKILL"] {