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/ui/window/named_parameters.rs

@@ -88,7 +88,7 @@ fn used_by_cell<'a>(scene: &Scene, name: &str) -> Element<'a, Message> {
 }
 
 /// Which column of a parameter row a text edit targets.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum ParamField {
     Name,
     Formula,

src/ui/window/options.rs

@@ -51,6 +51,7 @@ pub fn view_window<'a>(
     default_save_format: &'a str,
     file_assoc_enabled: bool,
     write_dwg_native_constraints: bool,
+    show_constraint_values: bool,
     ui_theme: &'a UiThemeConfig,
     theme_color_inputs: &'a [String; 6],
     language: crate::i18n::Language,
@@ -230,6 +231,23 @@ pub fn view_window<'a>(
         ))
         .size(11)
         .width(sizing.width),
+        Space::new().height(14),
+        row![
+            iced::widget::checkbox(show_constraint_values)
+                .on_toggle(Message::ShowConstraintValuesChanged)
+                .size(15),
+            text(crate::t!("Show values and parameter names on constraint markers")).size(12),
+        ]
+        .spacing(8)
+        .align_y(iced::Center),
+        Space::new().height(6),
+        text(crate::t!(
+            "On by default. Turn off to show just the constraint glyph in the viewport — \
+             enough to see that a constraint is present — without the driven value or named \
+             parameter text covering nearby geometry."
+        ))
+        .size(11)
+        .width(sizing.width),
     ]
     .spacing(0)
     .width(sizing.width);