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/automation.rs

@@ -302,6 +302,7 @@ impl OpenCADStudio {
                 let i = self.active_tab;
                 self.tabs[i].scene.document = acadrust::CadDocument::new();
                 self.tabs[i].scene.sketch_constraints.clear();
+                self.tabs[i].scene.named_parameters = crate::scene::named_parameters::ParameterTable::new();
                 self.tabs[i].scene.deselect_all();
                 self.tabs[i].current_path = None;
                 // The headless session starts on the welcome (Start) tab, which
@@ -331,6 +332,7 @@ impl OpenCADStudio {
                         let i = self.active_tab;
                         self.tabs[i].scene.document = doc;
                         self.tabs[i].scene.load_sketch_constraints_from_document();
+                        self.tabs[i].scene.load_named_parameters_from_document();
                         self.tabs[i].scene.deselect_all();
                         crate::app::style_ops::ensure_standard_styles(
                             &mut self.tabs[i].scene.document,
@@ -1063,6 +1065,7 @@ mod tests {
             "CHANGELOG",
             "CUI",
             "ALIASEDIT",
+            "OPTIONS",
         ];
         for cmd in standalone {
             assert!(