Add named parameters, UI scale setting, OPTIONS/OP dispatch, and help manual

Named parameters (docs/named_parameters_design.md, stages 3-4):
- ParameterTable (src/scene/named_parameters.rs): a document-wide table of
  named formulas, each resolved through a shared expression evaluator with
  cross-parameter references, cycle detection, and division-by-zero/parse
  error reporting. SketchConstraint::driving_param becomes DrivingValue
  (Literal | Named) instead of a bare f64, resolved at solve time by
  sketch_solve::build_constraint via Scene::named_parameters.
- Persistence (src/scene/named_parameters_persist.rs): same lazy XRecord
  model sketch_persist.rs already established, synced around open/save.
  Bumps the sketch-constraint XRecord's own format version (1 -> 2) since
  DrivingValue's shape doesn't line up with a version-1 blob's bytes.
- PARAMETERS command and its editor (src/ui/window/named_parameters.rs):
  add/remove/edit rows, live validation, duplicate-name detection: Apply
  defines the table and re-solves every constraint scope with a
  named-parameter-driven constraint (command_driver.rs). Distance/Angle
  constraint prompts (modules/draw/constrain/value.rs) now accept either a
  literal or an existing parameter's name, mirroring how the constraint's
  own value prompt already worked.
- Undo scope-down carried over from this project's own established pattern
  (constraint-system stage 9): the geometry an Apply moves is one undo
  step, but the parameter table edit itself has no undo entry yet — a
  documented follow-up, not a silent gap.

UI scale setting (src/app/settings.rs, app/mod.rs, ui/window/options.rs):
  a whole-interface scale percentage (50-200%), wired directly into iced's
  own per-window scale_factor hook rather than per-panel font-size
  plumbing, so it applies immediately to every panel, dialog, and the
  ribbon at once. Persisted alongside the existing lineweight-display-scale
  setting.

OPTIONS/OP command dispatch (app/commands/view.rs, app/commands/mod.rs):
  OPTIONS was registered for autocomplete but had no dispatch arm — the
  Start-tab welcome screen's own button was the only way to reach it.
  Found live-testing the UI-scale setting, unrelated to that feature
  itself. "OP" is AutoCAD's own short alias.

help/: a new self-contained static user-help manual (manual.md as the
authored source, tools/build_help.py splits it into the committed static
HTML pages plus search index) — not yet wired into the application.

64 new/updated tests across the named-parameters and constraint-prompt
changes.

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

src/app/update/mod.rs

@@ -5861,6 +5861,43 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            // ── Named Parameters (PARAMETERS) ─────────────────────────────────
+            Message::NamedParametersOpen => {
+                let i = self.active_tab;
+                self.named_parameter_editor_rows = self.tabs[i]
+                    .scene
+                    .named_parameters()
+                    .iter()
+                    .map(|p| crate::ui::window::named_parameters::ParamEditorRow { name: p.name.clone(), formula: p.source.clone() })
+                    .collect();
+                self.active_modal = Some(super::ModalKind::NamedParameters);
+                Task::none()
+            }
+            Message::NamedParametersInput { idx, field, value } => {
+                use crate::ui::window::named_parameters::ParamField;
+                if let Some(row) = self.named_parameter_editor_rows.get_mut(idx) {
+                    match field {
+                        ParamField::Name => row.name = value,
+                        ParamField::Formula => row.formula = value,
+                    }
+                }
+                Task::none()
+            }
+            Message::NamedParametersAdd => {
+                self.named_parameter_editor_rows.push(crate::ui::window::named_parameters::ParamEditorRow::default());
+                Task::none()
+            }
+            Message::NamedParametersRemove(idx) => {
+                if idx < self.named_parameter_editor_rows.len() {
+                    self.named_parameter_editor_rows.remove(idx);
+                }
+                Task::none()
+            }
+            Message::NamedParametersApply => {
+                self.apply_named_parameter_editor_rows();
+                Task::none()
+            }
+
             // ── Options / About windows ───────────────────────────────────
             Message::OptionsOpen => {
                 self.active_modal = Some(super::ModalKind::Options);
@@ -5900,6 +5937,12 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::UiScaleChanged(value) => {
+                self.ui_scale = value.clamp(50, 200);
+                self.persist_settings_if_changed();
+                Task::none()
+            }
+
             Message::RibbonLabelFontSizeChanged(value) => {
                 self.ribbon.set_label_font_size(value);
                 self.save_config();