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 <[email protected]>

src/app/update/file.rs

@@ -368,6 +368,7 @@ impl OpenCADStudio {
             cursor_type: self.cursor_type,
             crosshair_color: self.crosshair_color,
             lineweight_display_scale: self.lineweight_display_scale,
+            ui_scale: self.ui_scale,
             isometric_drafting: self.isometric_drafting,
             iso_plane: self.iso_plane,
             snap_angle_deg: self.snap_angle_deg,
@@ -425,6 +426,7 @@ impl OpenCADStudio {
             .map(crate::app::config::rgb_to_hex)
             .unwrap_or_default();
         self.lineweight_display_scale = s.lineweight_display_scale.clamp(25, 200);
+        self.ui_scale = s.ui_scale.clamp(50, 200);
         self.isometric_drafting = s.isometric_drafting;
         self.iso_plane = s.iso_plane;
         self.snap_angle_deg = if s.snap_angle_deg.is_finite() {
@@ -1394,6 +1396,9 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 // Design doc §8 stage 4: read back any persisted sketch
                 // constraint sets right after the document is installed.
                 self.tabs[i].scene.load_sketch_constraints_from_document();
+                // named_parameters_design.md stage 2: same load-time hook,
+                // for the document-wide parameter table.
+                self.tabs[i].scene.load_named_parameters_from_document();
                 self.tabs[i].active_layer = self.tabs[i]
                     .scene
                     .document
@@ -1660,6 +1665,9 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
         // Design doc §8 stage 4: write any sketch constraint sets into the
         // document right before it's serialized.
         self.tabs[i].scene.materialize_sketch_constraints_for_save();
+        // named_parameters_design.md stage 2: same save-time hook, for the
+        // document-wide parameter table.
+        self.tabs[i].scene.materialize_named_parameters_for_save();
     }
 
     #[cfg(not(target_arch = "wasm32"))]
@@ -2611,6 +2619,7 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                     self.stamp_header_sysvars(i);
                     self.sync_solid_models_for_save(i);
                     self.tabs[i].scene.materialize_sketch_constraints_for_save();
+                    self.tabs[i].scene.materialize_named_parameters_for_save();
                     let tab_id = self.tabs[i].id;
                     let bounds = crate::ui::wrap_bar::dropdown_bounds(
                         crate::app::view::VIEWPORT_CAPTURE_BOUNDS_ID,