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/scene/sketch_persist.rs

@@ -48,7 +48,15 @@ fn remove_xrecord(document: &mut CadDocument, owner: Handle, key: &str) {
 /// or panicking — design doc §5.2's explicit call-out, and §7's flagged
 /// "no migration path designed yet": bumping this is the signal a real
 /// migration is needed, not a substitute for one.
-const FORMAT_VERSION: u8 = 1;
+///
+/// Bumped 1 -> 2 by `named_parameters_design.md` stage 3:
+/// `SketchConstraint::driving_param` changed from a bare `Option<f64>` to
+/// `Option<named_parameters::DrivingValue>` (an enum, with its own
+/// discriminant), so a version-1 blob's bytes no longer line up with the
+/// current struct shape — exactly the "detected and gracefully skipped"
+/// case this version byte exists for, not something to migrate (no
+/// migration path exists yet, per the call-out above).
+const FORMAT_VERSION: u8 = 2;
 
 fn encode(set: &SketchConstraintSet) -> Option<Vec<u8>> {
     let mut bytes = vec![FORMAT_VERSION];
@@ -192,7 +200,11 @@ mod tests {
             vec![SketchRef::point(a, 1), SketchRef::point(b, 0)],
             None,
         );
-        scene.sketch_constraint_set_mut(SketchScope::ModelSpace).add(ConstraintKind::Distance, vec![SketchRef::whole(b)], Some(12.5));
+        scene.sketch_constraint_set_mut(SketchScope::ModelSpace).add(
+            ConstraintKind::Distance,
+            vec![SketchRef::whole(b)],
+            Some(crate::scene::named_parameters::DrivingValue::Literal(12.5)),
+        );
 
         scene.materialize_sketch_constraints_for_save();
         let bytes = crate::io::save_to_bytes(&scene.document, ext, scene.document.version)
@@ -213,7 +225,7 @@ mod tests {
         assert_eq!(restored.constraints.len(), 2);
         assert_eq!(restored.constraints[0].kind, ConstraintKind::Coincident);
         assert_eq!(restored.constraints[1].kind, ConstraintKind::Distance);
-        assert_eq!(restored.constraints[1].driving_param, Some(12.5));
+        assert_eq!(restored.constraints[1].driving_param, Some(crate::scene::named_parameters::DrivingValue::Literal(12.5)));
     }
 
     #[test]
@@ -222,7 +234,7 @@ mod tests {
         assert_eq!(restored.constraints.len(), 2);
         assert_eq!(restored.constraints[0].kind, ConstraintKind::Coincident);
         assert_eq!(restored.constraints[1].kind, ConstraintKind::Distance);
-        assert_eq!(restored.constraints[1].driving_param, Some(12.5));
+        assert_eq!(restored.constraints[1].driving_param, Some(crate::scene::named_parameters::DrivingValue::Literal(12.5)));
     }
 
     #[test]