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/mod.rs

@@ -36,6 +36,8 @@ mod group_layer;
 mod layout;
 mod limits;
 mod modify;
+pub mod named_parameters;
+mod named_parameters_persist;
 mod mspace;
 mod page_setup;
 mod paper;
@@ -1934,6 +1936,17 @@ pub struct Scene {
     /// touches directly, materialized into `document.objects` only right
     /// before a save and read back right after a load.
     pub(crate) sketch_constraints: Vec<sketch_constraints::SketchConstraintSet>,
+    /// Document-wide named-parameter/expression table (`docs/
+    /// named_parameters_design.md`). One table per document (not per-scope,
+    /// unlike `sketch_constraints` — a parameter is meant to be shared
+    /// across the whole drawing), lives on `Scene` for the same reason
+    /// `sketch_constraints` does: co-located with the `document: CadDocument`
+    /// it's synced against. Synced with a single document-level
+    /// `OCS_NAMED_PARAMETERS` XRecord (hung off `document.header.
+    /// named_objects_dict_handle`, not a per-`BlockRecord` one) around
+    /// open/save (`named_parameters_persist.rs`, same "lazy" model as
+    /// `sketch_persist.rs`).
+    pub(crate) named_parameters: named_parameters::ParameterTable,
     /// Tessellated block definitions in block-local coords, keyed by render
     /// background and block epoch. Model and Paper adapt black/white colours
     /// differently; retaining both variants prevents a full block rebuild on
@@ -2167,6 +2180,7 @@ impl Scene {
             dependency_index_cache: RefCell::new(None),
             associative_hatch_source_cache: RefCell::new(None),
             sketch_constraints: Vec::new(),
+            named_parameters: named_parameters::ParameterTable::new(),
             has_associative_centers: std::cell::Cell::new(None),
             block_defn_cache: RefCell::new(HashMap::default()),
             entity_index_cache: RefCell::new(None),