Publish 3MF: support mixed filaments and per-extruder slot selection

- Publish mixed-filament slots as whole units: serialize the filament_mixed_* definition into project_config on import, grow the receiver's parallel arrays in lockstep, and report unappliable definitions as skipped instead of dropping them silently

- Per-extruder printer selection: one inner tab per extruder, rows keyed by full "#N" ids; single-extruder receivers collapse variants onto their slot (first applied, rest skipped), multi-extruder receivers override element-wise

- New per-slot "Enable" toggle gating what gets published; enabling a mix auto-enables + Full Publishes its components

- Mixed page previews: fixed-size ratio bar, ternary triangle (3 components) and Material Ratio vs Model Height graph (gradients), always visible regardless of Enable

- Tab strip shows full swatch compositions with adjustable spacing; barycentric helpers shared via FilamentBitmapUtils

src/libslic3r/PublishSettings.cpp

@@ -83,6 +83,23 @@ const std::set<std::string>& publish_structural_keys()
     return structural_keys;
 }
 
+const std::set<std::string>& publish_mixed_keys()
+{
+    // Must match PresetBundle's s_project_options mixed-color group (PresetBundle.cpp): these
+    // are project-level parallel per-slot arrays, not filament-preset options, so the import
+    // material pass applies them into project_config instead of a filament preset config.
+    static const std::set<std::string> mixed_keys = {
+        "filament_is_mixed",
+        "filament_mixed_components",
+        "filament_mixed_sublayer_ratios",
+        "filament_mixed_gradient",
+        "filament_mixed_gradient_range",
+        "filament_mixed_gradient_curve",
+        "filament_mixed_gradient_per_part"
+    };
+    return mixed_keys;
+}
+
 // The printer tab's "Retraction" optgroup (TabPrinter::build_fff, Tab.cpp), in tab order.
 // KEEP IN SYNC with that optgroup: the published-3MF printer allowlist is built from these
 // lists, so any key shown there must be publishable here (and vice versa).