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>

help/search-index.js

@@ -0,0 +1 @@
+window.OCS_HELP_INDEX = [{"title":"What OpenCAD Studio does","chapter":"Getting Started","url":"getting-started.html#what-opencad-studio-does","text":"What OpenCAD Studio does OpenCAD Studio is a desktop and web CAD application for opening, creating, editing, visualizing, and saving two- and three-dimensional drawings. It works with DWG and DXF drawings and provides command-line, ribbon, automation, and plugin-oriented workflows."},{"title":"A first drawing","chapter":"Getting Started","url":"getting-started.html#a-first-drawing","text":"A first drawing Create a new drawing or open an existing DWG or DXF file. Confirm the active workspace, units, layer, and object-snap settings. Create geometry with a drawing command such as Line , Circle , or Polyline . Edit the result with grips or a modification command. Save the drawing under a new name until its compatibility has been verified. Data safety: Keep an untouched copy of important third-party drawings. Confirm geometry, layouts, external references, and plotted output before replacing the original file."},{"title":"How to use this manual","chapter":"Getting Started","url":"getting-started.html#how-to-use-this-manual","text":"How to use this manual Use the chapter navigation for broad workflows, the search box for a command or concept, and the previous/next links for sequential reading. Command names, system variables, keys, and file extensions appear in monospaced text ."},{"title":"Create and open drawings","chapter":"Files and Drawings","url":"files-drawings.html#create-and-open-drawings","text":"Create and open drawings Use New for a blank document and Open for an existing DWG, DXF, backup, or autosave drawing. After opening a file from another CAD system, inspect its units, layers, blocks, fonts, raster references, layouts, and model extents."},{"title":"Save and Save As","chapter":"Files and Drawings","url":"files-drawings.html#save-and-save-as","text":"Save and Save As Save updates the current drawing. Save As changes its path, format, or version. Prefer Save As when validating interoperability or recovering an older drawing. A successful write does not replace visual inspection of a reopened copy."},{"title":"Recovery and backups","chapter":"Files and Drawings","url":"files-drawings.html#recovery-and-backups","text":"Recovery and backups OpenCAD Studio recognizes drawing backups and autosave-style files where the underlying data is DWG or DXF. If strict loading rejects malformed entities, use the recovery workflow and review what was discarded before continuing work."},{"title":"External resources","chapter":"Files and Drawings","url":"files-drawings.html#external-resources","text":"External resources Raster images, underlays, fonts, materials, and external references may depend on paths outside the drawing. Keep referenced files together when moving a project and verify that relative paths still resolve."},{"title":"Model space and paper space","chapter":"Workspace and Navigation","url":"workspace-navigation.html#model-space-and-paper-space","text":"Model space and paper space Model space is the primary drawing environment. Paper-space layouts arrange views for plotting through layout viewports. Always confirm the active space before drawing or editing."},{"title":"Pan, zoom, and orbit","chapter":"Workspace and Navigation","url":"workspace-navigation.html#pan-zoom-and-orbit","text":"Pan, zoom, and orbit Use pan and zoom to navigate without changing drawing geometry. Use orbit and view controls for three-dimensional inspection. Zoom Extents is useful when the drawing appears empty or geometry is far from the expected location."},{"title":"Multiple model viewports","chapter":"Workspace and Navigation","url":"workspace-navigation.html#multiple-model-viewports","text":"Multiple model viewports Model viewports can show different views of the same document. Grid, snap, camera, and render mode may vary by viewport; confirm which viewport is active before entering coordinates."},{"title":"Selection and object snaps","chapter":"Workspace and Navigation","url":"workspace-navigation.html#selection-and-object-snaps","text":"Selection and object snaps Selection identifies the objects a command will modify. Object snaps target precise geometric locations such as endpoints, midpoints, centers, and intersections. Zoom in and verify the snap marker when precision matters."},{"title":"Basic drawing commands","chapter":"Create and Edit Geometry","url":"create-edit-geometry.html#basic-drawing-commands","text":"Basic drawing commands Create lines, arcs, circles, ellipses, polylines, splines, points, text, hatches, and other supported entities from the ribbon or command line. Follow the active prompt: multi-step commands remain active until they complete or are cancelled."},{"title":"Modify geometry","chapter":"Create and Edit Geometry","url":"create-edit-geometry.html#modify-geometry","text":"Modify geometry Common modification workflows include move, copy, rotate, scaLine truncated