fix(alpine): build R with cairo, png, jpeg and tiff graphics devices (#9)

## 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

CLAUDE.md

@@ -10,4 +10,11 @@ CI pipelines (Crow, in `.crow/`) that build R binaries and upload them to Backbl
 ## Gotchas
 
 - **B2 requires authentication for the list-bucket API.** Anonymous GET only works for individual public-read objects — an empty bucket listing usually means missing credentials, not an empty bucket. This silently broke rebuild dedup once.
+- **R's configure silently disables graphics devices when headers are missing.**
+It never fails the build, so a missing `-dev` package yields a working 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; the binary has to be rebuilt.
+`builder/build.sh` therefore asserts `REQUIRED_CAPABILITIES` (set per Dockerfile) after `make install`.
+- **The `--with-x=no` on the Alpine images does not disable cairo.** R's bitmap and pangocairo checks run independently of `R_X11`, so `png()` and `svg()` work headless via cairo.
+- **Artifact filenames carry only the R version, not the builder image.**
+When a builder change alters the resulting binary, dedup would keep serving the stale artifact forever; set `FORCE_REBUILD` in `.crow/build.yaml` to rebuild and overwrite in place.
 - When a pipeline fails, fetch the Crow logs yourself instead of asking the user to paste them, and monitor reruns in the background until green.