## Motivation
R on the Alpine images was built without graphics support: `capabilities()` reports `cairo`, `png`, `jpeg` and `tiff` as `FALSE`, while Ubuntu reports all four `TRUE`.
The images install `R-dev` but never the graphics headers, and R's `configure` treats those as optional and does not fail when they are absent, so the build succeeds and ships an R that cannot draw.
Reproduced in a clean `alpine:3.22` container with the old package set:
```
Additional capabilities: PNG, NLS, ICU
Capabilities skipped: JPEG, TIFF, cairo
```
One correction to the original diagnosis: `--with-x=no` was not the cause, and libpng was in fact found.
R's bitmap and `pangocairo` checks run independently of `R_X11` (`configure.ac:2604` gates on `want_cairo`, not `use_X11`).
`capabilities("png")` needs a device *backend*, and with X11 off and cairo absent there was none, which is why all four read `FALSE` despite configure listing PNG.
So the fix is purely the missing `-dev` packages.
X11 stays off, which is correct for headless images, and cairo covers `png()` / `jpeg()` / `tiff()` / `svg()`.
## Changes
- `builder/Dockerfile.alpine-{320,321,322,323,324}` — install `pkgconf cairo-dev pango-dev fontconfig-dev freetype-dev libpng-dev jpeg-dev tiff-dev zlib-dev`; pass `--with-cairo --with-libpng --with-jpeglib --with-libtiff` explicitly so the intent sits next to `--with-x=no`; set `REQUIRED_CAPABILITIES`.
- `builder/build.sh` — add `verify_capabilities()` between `compile_r` and `package_r`. A build that loses a graphics device now fails instead of being packaged and uploaded. Opt-in via `REQUIRED_CAPABILITIES`, so every non-Alpine platform is untouched.
- `.crow/build.yaml` — 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.
- `CLAUDE.md` — record the three gotchas.
## Verification
Compiled R 4.5.1 in the patched `alpine-322` image, through the real `build.sh`:
```
jpeg png tiff tcltk X11 aqua
TRUE TRUE TRUE TRUE FALSE FALSE
NLS Rprof profmem cairo ICU long.double
TRUE TRUE TRUE TRUE TRUE TRUE
All required capabilities are present.
```
The devices actually render, not just report `TRUE` (`default bitmapType: cairo`):
```
png -> t.png (2154 bytes)
jpeg -> t.jpeg (4489 bytes)
tiff -> t.tiff (120160 bytes)
svg -> t.svg (9381 bytes)
```
The guard was exercised in both directions against that real build: it passes on the four graphics capabilities, and returns non-zero naming the missing entry when additionally asked for `X11`.
`crow lint .crow/` passes and `bash -n builder/build.sh` is clean.
## Behaviour change
Alpine R binaries gain the cairo, png, jpeg and tiff devices, and link against `libcairo`, `libpangocairo`, `libpng`, `libjpeg` and `libtiff`.
The `.apk` metadata already declared those runtime dependencies, so consumer images need no change.
Any Alpine build that loses a graphics device now fails the pipeline rather than uploading silently.
**Needs a follow-up after merge:** the first run must have `FORCE_REBUILD` set, otherwise dedup skips every existing Alpine version and the broken artifacts stay in the bucket.
Reviewed-on: #9
