Feature/ranking question type

[Sunilshah-7]

Add ranking question type

Implements the ranking proposal discussed in the issue.

API

Matches what we agreed in the issue:

sd_question(
  type  = 'ranking',
  id    = 'albums_ranked',
  label = "Rank these Taylor Swift albums from favorite to least favorite:",
  option = c(
    "Taylor Swift (2006)" = "swift",
    "Fearless (2008)"     = "fearless",
    "Speak Now (2010)"    = "speak_now",
    "Red (2012)"          = "red",
    "1989 (2014)"         = "1989"
  )
)

Stored as a single pipe-joined string in ranked order (swift|red|fearless|...), following the mc_multiple convention.

Implementation approach

No new dependency. ~120 lines of Pointer Events drag logic ininst/js/ranking.js rather than pulling in a library. Pointer Events give mouse and touch one code path (native HTML5 drag-and-drop is unusable here - it doesn't fire on
touch devices).

What I evaluated:

Option	Finding
jQuery UI (bundled in Shiny)	Does include sortable, but isn't loaded by default and Shiny's build is 253KB — 5.6× SortableJS
shinyjqui	Wraps that same jQuery UI: new dependency and the 253KB
Vendoring SortableJS 1.15.6	45KB raw / ~15KB gzipped. Verified the copy inside sortable is byte-identical to the upstream release
Hand-rolled Pointer Events	~120 lines, no new files beyond the ones the type needs anyway

Tradeoff as I see it: vendoring is tested and near-zero review/maintenance cost (upgrades are a file swap), but because include-resources.lua + --embed-resources inline everything in js_files, it would be baked into every rendered survey whether or not it uses a ranking question. Hand-rolling keeps the payload and the diff small and
matches the style of the existing inst/js/ files, but means surveydown owns drag-and-drop bug reports going forward — and sorting's cross-bin dragging in the follow-up is meaningfully harder to hand-roll than ranking's single-list reorder.

Files changed

File	What
inst/js/ranking.js (new)	Pointer Events drag, order reporting, restore handler, test hook
inst/css/surveydown.css	.sd-ranking-* styles; position numbers via CSS counters
inst/lua/include-resources.lua	Register ranking.js
R/question_types.R	qt_render_ranking(), qt_restore_ranking(), registry entry
R/config.R	Detect .sd-ranking-list, extract options, map class → ranking
R/ui.R	Roxygen docs only
man/sd_question.Rd	Regenerated
NEWS.md	Entry

Reproducible steps

git checkout feature/ranking-question-type
R CMD INSTALL .

# minimal test survey (survey.qmd + app.R shown below)
rm -rf test_survey/_survey test_survey/preview_data.csv
Rscript -e 'shiny::runApp("test_survey", port = 3838)'

Please let me know additional test cases and reviews for this feature.