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/modules/draw/constrain/value.rs

@@ -57,7 +57,7 @@ pub mod distance_tool {
         ToolDef {
             id: "DCONSTRAINT",
             label: "Distance",
-            icon: IconKind::Glyph("↔"),
+            icon: IconKind::Svg(include_bytes!("../../../../assets/icons/constrain/distance.svg")),
             event: ModuleEvent::Command("DCONSTRAINT".to_string()),
         }
     }
@@ -69,7 +69,7 @@ pub mod angle_tool {
         ToolDef {
             id: "ACONSTRAINT",
             label: "Angle",
-            icon: IconKind::Glyph("∠"),
+            icon: IconKind::Svg(include_bytes!("../../../../assets/icons/constrain/angle.svg")),
             event: ModuleEvent::Command("ACONSTRAINT".to_string()),
         }
     }