fix(alpine): build R with cairo, png, jpeg and tiff graphics devices

The Alpine builder images install R-dev but never the graphics headers.
R's configure treats those as optional and does not fail when they are
absent, so the build succeeds and ships an R whose capabilities() reports
cairo, png, jpeg and tiff as FALSE. Installing the runtime libraries on
the target system afterwards cannot bring compiled-out devices back.

Note that --with-x=no was not the cause. R's bitmap and pangocairo checks
run independently of R_X11, and libpng was in fact found. capabilities("png")
needs a device backend, and with X11 off and cairo absent there was none,
which is why all four read FALSE. X11 stays off: these are headless images
and cairo covers png()/jpeg()/tiff()/svg().

- Install pkgconf, cairo-dev, pango-dev, fontconfig-dev, freetype-dev,
  libpng-dev, jpeg-dev, tiff-dev and zlib-dev in every Alpine builder image.
- Pass --with-cairo, --with-libpng, --with-jpeglib and --with-libtiff
  explicitly so the intent is visible next to --with-x=no.
- Assert REQUIRED_CAPABILITIES after make install, so a build that loses a
  device fails instead of being packaged and uploaded. Opt-in per Dockerfile,
  leaving the other platforms untouched.
- Add a FORCE_REBUILD escape hatch to the dedup step. Artifact filenames
  carry only the R version, so the HEAD probe would otherwise keep skipping
  every Alpine version and serving the stale broken binaries.
- Record the three gotchas in CLAUDE.md.

Verified by compiling R 4.5.1 in the patched alpine-322 image: jpeg, png,
tiff and cairo are TRUE, default bitmapType is cairo, and all four devices
render. The guard was exercised in both directions against that build.

builder/build.sh

@@ -209,6 +209,47 @@ local({
 EOF
 }
 
+# verify_capabilities()
+# Guard against a graphics device being silently compiled out. R's configure
+# only enables the cairo/png/jpeg/tiff devices when the matching headers were
+# present at compile time, and it does not fail when they are missing, so a
+# missing -dev package produces a working R that simply cannot draw. Once that
+# tarball is published, installing the runtime libraries on the target system
+# cannot bring the devices back; the build has to be redone.
+#
+# Dockerfiles opt in by setting REQUIRED_CAPABILITIES to a comma-separated list
+# of capabilities() names, e.g. "cairo,png,jpeg,tiff".
+verify_capabilities() {
+  if [ -z "${REQUIRED_CAPABILITIES}" ]; then
+    return 0
+  fi
+
+  echo "Verifying required R capabilities: ${REQUIRED_CAPABILITIES}"
+
+  # The full capabilities() table goes to stderr so it shows up in the build
+  # log; stdout carries only the space-separated list of missing entries.
+  local missing
+  missing=$("${R_INSTALL_PATH}/bin/Rscript" --vanilla -e '
+    required <- trimws(strsplit(Sys.getenv("REQUIRED_CAPABILITIES"), ",")[[1]])
+    required <- required[nzchar(required)]
+    caps <- capabilities()
+    writeLines(capture.output(print(caps)), con = stderr())
+    ok <- (required %in% names(caps)) & (unname(caps[required]) %in% TRUE)
+    cat(paste(required[!ok], collapse = " "), "\n", sep = "")
+  ') || {
+    echo "ERROR: could not query capabilities() from the freshly built R" >&2
+    return 1
+  }
+
+  if [ -n "${missing}" ]; then
+    echo "ERROR: R ${R_VERSION} for ${OS_IDENTIFIER} is missing required capabilities: ${missing}" >&2
+    echo "ERROR: install the matching -dev packages in builder/Dockerfile.${OS_IDENTIFIER} and rebuild." >&2
+    return 1
+  fi
+
+  echo "All required capabilities are present."
+}
+
 # check for packager script
 ## If it exists this build is ready for packaging with nFPM, so run the script
 ## else do nothing
@@ -236,6 +277,7 @@ set_up_environment
 fetch_r_source $R_VERSION
 patch_r $R_VERSION
 compile_r $R_VERSION
+verify_capabilities
 package_r $R_VERSION
 archive_r
 upload_r $R_VERSION