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/mod.rs

@@ -504,6 +504,12 @@ pub(super) struct OpenCADStudio {
     crosshair_color_input: String,
     /// Model-space lineweight preview scale, in percent (25..=200).
     lineweight_display_scale: i32,
+    /// Whole-interface scale, in percent (50..=200) — drives iced's own
+    /// per-window `scale_factor` (`run`'s `.scale_factor(...)` hook), so it
+    /// resizes every panel, dialog, the ribbon, and their text uniformly
+    /// rather than needing a separate font-size knob threaded through each
+    /// panel's own view code.
+    ui_scale: i32,
     /// Isometric drafting state and active axis pair.
     isometric_drafting: bool,
     iso_plane: settings::IsoPlane,
@@ -1018,6 +1024,15 @@ pub(super) struct OpenCADStudio {
     /// edited. Seeded from `command_aliases` on open, committed back on close.
     alias_editor_rows: Vec<(String, String)>,
 
+    // ── Named Parameters (docs/named_parameters_design.md, stage 4) ────────
+    /// Working buffer for the PARAMETERS modal. Unlike `alias_editor_rows`,
+    /// this isn't a copy of a separate app-level store — the real state
+    /// lives per-document at `Scene::named_parameters`; this buffer is
+    /// seeded from the active tab's table on open and only written back to
+    /// it on Apply (`apply_named_parameter_editor_rows`,
+    /// `src/app/named_parameters.rs`).
+    named_parameter_editor_rows: Vec<crate::ui::window::named_parameters::ParamEditorRow>,
+
     // ── Layout Manager Panel ──────────────────────────────────────────────
     layout_manager_selected: String,
     layout_manager_rename_buf: String,
@@ -1695,6 +1710,7 @@ pub enum ModalKind {
     AttributeEditor,
     LayerDeleteWarning,
     Aliases,
+    NamedParameters,
     ScaleManager,
     /// Add / remove the annotation scales a single selected object has a
     /// per-object representation for.
@@ -1937,6 +1953,8 @@ pub enum Message {
     CursorTypeChanged(settings::CursorType),
     /// Set the model-space lineweight preview scale from Options.
     LineweightDisplayScaleChanged(i32),
+    /// Set the whole-interface scale (`ui_scale`) from Options.
+    UiScaleChanged(i32),
     /// Set the ribbon's large-button icon-label font size from Options.
     RibbonLabelFontSizeChanged(i32),
     /// Set the ribbon's panel-title ("Draw", "Modify", …) font size from Options.
@@ -2706,6 +2724,24 @@ pub enum Message {
     AliasEditorRemove(usize),
     /// Commit the edited rows to the alias table (Apply button); stays open.
     AliasEditorApply,
+    // ── Named Parameters (PARAMETERS) ───────────────────────────────────
+    /// Open the named-parameter editor, seeding rows from the active tab's
+    /// `Scene::named_parameters`.
+    NamedParametersOpen,
+    /// Live edit of the name or formula in row `idx`.
+    NamedParametersInput {
+        idx: usize,
+        field: crate::ui::window::named_parameters::ParamField,
+        value: String,
+    },
+    /// Append a blank parameter row.
+    NamedParametersAdd,
+    /// Remove parameter row `idx`.
+    NamedParametersRemove(usize),
+    /// Commit the edited rows to `Scene::named_parameters` (Apply button)
+    /// and re-solve every constraint that reads a named parameter; stays
+    /// open.
+    NamedParametersApply,
     // ── About window ────────────────────────────────────────────────────
     AboutOpen,
     /// Close whatever in-canvas modal dialog is open (Plan B).
@@ -3378,6 +3414,7 @@ impl OpenCADStudio {
             crosshair_color: None,
             crosshair_color_input: String::new(),
             lineweight_display_scale: 100,
+            ui_scale: 100,
             isometric_drafting: false,
             iso_plane: settings::IsoPlane::Left,
             snap_angle_deg: 0.0,
@@ -3579,6 +3616,7 @@ impl OpenCADStudio {
             // Command aliases (populated from ocad.pgp just after construction)
             command_aliases: rustc_hash::FxHashMap::default(),
             alias_editor_rows: Vec::new(),
+            named_parameter_editor_rows: Vec::new(),
             // Layout Manager
             layout_manager_selected: "Model".to_string(),
             layer_state_selected: None,
@@ -3978,6 +4016,7 @@ pub fn run() -> iced::Result {
         ..iced::Settings::default()
     })
     .subscription(OpenCADStudio::subscription)
+    .scale_factor(|state: &OpenCADStudio, _window: window::Id| state.ui_scale as f32 / 100.0)
     .title(|state: &OpenCADStudio, window_id: window::Id| {
         let _ = window_id; // all dialogs are in-canvas modals now
         if let Some(tab) = state.tabs.get(state.active_tab) {
@@ -4022,6 +4061,7 @@ pub fn run_web() -> iced::Result {
         OpenCADStudio::view_main,
     )
     .subscription(OpenCADStudio::subscription)
+    .scale_factor(|state: &OpenCADStudio| state.ui_scale as f32 / 100.0)
     .title(|_state: &OpenCADStudio| {
         concat!("Open CAD Studio ", env!("OCS_APP_VERSION")).to_string()
     })