Add Properties-panel Constraints/Parameters sections, a constraint-pill

value toggle, and redesigned constraint-ribbon icons

Properties panel:
- New "Parameters" section (shown with no selection) listing every named
  parameter with inline name/formula editing, add, and delete, backed by
  new PropValue::ParamRow/ParamAddRow and command_driver handlers
  (resolve_named_parameter_edit, on_prop_param_commit/_delete/_add_new).
  A fresh parameter's formula now defaults to "1", not "0" — a 0-length
  seed value is a genuine solver singularity for a Distance constraint
  driven by that parameter, with no defined direction to grow back out of.
- New "Constraints" section (shown for a single selected entity) listing
  every constraint touching it as a clickable PropValue::EntityLink that
  selects the linked entities in the viewport, danger-tinted when the
  constraint conflicts.

Constraint pills:
- New Options > General toggle, "Show values and parameter names on
  constraint markers" (on by default, persisted). Off, every pill shows
  just its bare glyph instead of glyph+driven-value/parameter-name — the
  glyph alone is enough to see a constraint is present, without the value
  text covering nearby geometry on a dense sketch.

Constraint-ribbon icons:
- Replaced all 18 unicode/letter glyph icons (including the plain "F",
  "L", "M", "S" letters) with proper SVGs under assets/icons/constrain/,
  matching the Modify section's two-color style (#B4B6B9 white / #6DB7ED
  blue, consistent stroke width). Tangent reuses the Draw section's
  white-arc-plus-blue-dots motif.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -839,10 +839,12 @@ bg={bg_ms:.1}ms n={view_count}"
                                 )?;
                                 let point = iced::Point::new(bounds.x + screen.x, bounds.y + screen.y);
                                 let is_conflicting = set.conflicts.iter().any(|(id, _)| *id == c.id);
-                                point
-                                    .x
-                                    .is_finite()
-                                    .then(|| (point, crate::scene::sketch_constraints::glyph_label(c), is_conflicting))
+                                let label = if self.show_constraint_values {
+                                    crate::scene::sketch_constraints::glyph_label(c)
+                                } else {
+                                    c.kind.glyph_symbol().to_string()
+                                };
+                                point.x.is_finite().then(|| (point, label, is_conflicting))
                             })
                             .collect()
                     }

src/app/view/modal.rs

@@ -198,6 +198,7 @@ impl OpenCADStudio {
                         &self.default_save_format,
                         self.file_assoc_enabled,
                         self.write_dwg_native_constraints,
+                        self.show_constraint_values,
                         &self.ui_theme,
                         &self.theme_color_inputs,
                         self.language,