fix: .Rbuildignore excludes stray Rplots.pdf (top-level NOTE) (#135)

R CMD check flagged a non-standard top-level file `Rplots.pdf`. It's a
default-device artefact created when a plot is drawn with no open device
(interactive smoke-tests, testthat runs). It's already gitignored but was
not build-ignored, so R CMD build copied it into the tarball.

Add an unanchored `Rplots\.pdf$` rule so the artefact is excluded from the
tarball at the root or under tests/ regardless of who left one behind.
Verified: with strays present at both locations, the built tarball
contains no Rplots.pdf.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: ehrlinger

.Rbuildignore

@@ -54,3 +54,6 @@ framed.sty
 ^vignettes/\.quarto$
 ^vignettes/.*_files$
 ^vignettes/.*\.html$
+# Stray default-device plot artefact (created by plotting with no open device),
+# at the root or under tests/ — never ship it.
+Rplots\.pdf$