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/ui/window/options.rs

@@ -41,6 +41,7 @@ pub fn view_window<'a>(
     crosshair_color: Option<[u8; 3]>,
     crosshair_color_input: &'a str,
     lineweight_display_scale: i32,
+    ui_scale: i32,
     ribbon_label_font_size: i32,
     ribbon_group_title_font_size: i32,
     model_space: &'a crate::app::config::ModelSpaceThemeConfig,
@@ -349,6 +350,24 @@ pub fn view_window<'a>(
         });
 
     let mut display = column![
+        text(crate::t!("UI Scale")).size(15),
+        Space::new().height(10),
+        row![
+            text(crate::t!("Overall size")).size(12).width(150),
+            slider(50..=200, ui_scale.clamp(50, 200), Message::UiScaleChanged)
+                .step(5)
+                .width(Fill),
+            text(format!("{}%", ui_scale.clamp(50, 200))).size(11).width(44),
+        ]
+        .spacing(12)
+        .align_y(iced::Center),
+        Space::new().height(8),
+        text(crate::t!(
+            "Scales the whole interface at once — every panel, dialog, the ribbon, and their text — rather than one widget's font size. Takes effect immediately."
+        ))
+        .size(11)
+        .width(sizing.width),
+        Space::new().height(24),
         text(crate::tr!("options", "theme-section")).size(15),
         Space::new().height(10),
         row![