fix(lint): enforce accessible SVG viewBox and title rules (#123)

* fix(lint): enforce viewBox presence and title length cap

- Add a11y check: non-decorative <svg> must carry a viewBox attribute
  with exactly four numeric tokens and positive width/height dimensions.
- Add a11y check: <title> text must be 60 characters or fewer, closing
  the gap between the CONTRIBUTING.md spec and linter enforcement.
- Update VALID_SVG in test-lint-a11y.py to include viewBox so all
  existing passing cases remain valid after the new checks land.
- Add three adversarial cases: missing-viewbox, invalid-viewbox,
  title-too-long — all verified against the gate suite (47/47 OK).

* fix(lint): accept SVG comma separators and reject non-finite viewBox values

- Re-split viewBox on [\s,]+ so comma-separated values (e.g. 0,0,800,600)
  are accepted as valid SVG
- Guard each component with math.isfinite() so values like inf and nan are
  rejected rather than passing the > 0 size check
- Add adversarial test cases: viewbox-comma-separated (pass),
  viewbox-nonfinite-width, viewbox-nonfinite-height (both fail)
- Bump plugin manifests to 2.4.1

* fix(lint): reject viewBox numbers with Python-only underscore separators

* test(lint): cover remaining viewBox cases

---------

Co-authored-by: Rupam Pal <rupam.pal@euromonitor.com>

.factory-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "diagram-design",
   "description": "Create branded architecture, IT current-state, flowchart, sequence, state machine, ER/data model, timeline, swimlane, quadrant, radar/spider, loop/flywheel, nested, tree, org chart, layer stack, Venn, pyramid/funnel, treemap, bar, line, Gantt and scatter charts, high-level, process, medallion, data flow, DP integration, DP security matrix, sankey, fishbone, Wardley map, kanban, user journey, deployment, dependency graph, UML class, story map, or database schema diagrams as standalone HTML/SVG/PNG. Redraw .drawio/.drawio.png/.drawio.svg or Mermaid .mmd sources at a chosen size/detail; onboard brand tokens from a website; add semantic patterns, callouts, accessible motion, or sketchy/hand-drawn styling.",
-  "version": "2.5.15",
+  "version": "2.5.16",
   "author": {
     "name": "Cathryn Lavery",
     "url": "https://github.com/cathrynlavery"