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/commands/mod.rs

@@ -292,6 +292,8 @@ pub fn start_allowed(cmd: &str) -> bool {
             | "PERF"
             | "CUI"
             | "ALIASEDIT"
+            | "OPTIONS"
+            | "OP"
             | "CUILOAD"
             | "CUIIMPORT"
     )
@@ -312,6 +314,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "CLEANSCREEN",
         "CUI",
         "DSETTINGS",
+        "PARAMETERS",
         "GRID",
         "ISODRAFT",
         "ISOPLANE",

src/app/commands/view.rs

@@ -240,6 +240,25 @@ impl OpenCADStudio {
                 return Some(Task::done(Message::AliasEditorOpen));
             }
 
+            // ── OPTIONS — app preferences dialog ────────────────────────────
+            // Registered for autocomplete (`commands/mod.rs`'s "one-shot
+            // commands" list) but had no actual dispatch arm anywhere — the
+            // Start-tab welcome screen's "Options" button emits
+            // `Message::OptionsOpen` directly and was the only way to reach
+            // it. Found while live-testing the new UI-scale setting (nothing
+            // to do with that feature itself, just discovered along the
+            // way). "OP" is AutoCAD's own short alias for this command.
+            "OPTIONS" | "OP" => {
+                return Some(Task::done(Message::OptionsOpen));
+            }
+
+            // ── PARAMETERS — named-parameter/expression table editor ───────
+            // `docs/named_parameters_design.md` stage 4. AutoCAD's own
+            // command name for the equivalent feature (Parameters Manager).
+            "PARAMETERS" => {
+                return Some(Task::done(Message::NamedParametersOpen));
+            }
+
             // ── CUI — keyboard shortcut / key-binding editor ───────────────
             "CUI" => {
                 return Some(Task::done(Message::ShortcutsPanelOpen));