Make lago_report() an interactive HTML dashboard

[correspondMerchant]

What this changes

lago_report() now renders a self-contained interactive HTML dashboard instead
of a static report. The confidence set is drawn as a hover-enabled D3 plot, a
scatter for a two-component intervention with the recommended intervention
highlighted or a one-dimensional strip for a single component, and each
intervention component gets interactive total-cost and marginal-cost curves with
a hover read-out. For three or more components, or when jsonlite is not
installed, the report falls back to the static plot.lago() figure, and when
there is no confidence set the plot section is omitted.

The report stays a single offline file. The vendored D3 v7 library and a new
inst/js/lago-report.js are inlined into the HTML, so there is no CDN, server,
or extra widget framework, and the public API of lago_report() is unchanged.
Rendering now also uses jsonlite, added under Suggests, to serialize the result
for the client-side code.

The cost curves are labelled by the main components the cost vectors are indexed
by (display_components), so a model with interaction terms is labelled
correctly rather than by the longer intervention_components vector that carries
the backticked interaction entry.

Related issue

No linked issue.

Checklist

• Tests added or updated under tests/testthat/ (new behaviour has a test
  that fails without this change). Extended test-report.R with render tests
  for the interactive pieces and an interaction-terms regression test for the
  cost labels, which fails if the label fix is reverted. Added
  tests/js/test-report-math.js (24 assertions) and wired it into the
  js-test workflow.
• devtools::document() run if roxygen comments changed. Man page
  regenerated, no pending diff.
• devtools::check() passes locally. Not run locally, following the
  "no local builds unless asked" convention. The full testthat suite passes
  via devtools::test(), both D3-math node tests pass, and CI runs
  R CMD check on the PR.
• NEWS.md updated with a one-line entry if this is a user-facing change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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