feat: add alpine-3.24 build config (#5)

## Summary

Adds an `alpine-3.24` build configuration, mirroring the existing `alpine-3.23` setup.

- `builder/Dockerfile.alpine-324` — `FROM alpine:3.24`, `OS_IDENTIFIER=alpine-324`, copies `package.alpine-324`
- `builder/package.alpine-324` — nfpm spec (identical to 323)
- `builder/docker-compose.yml` — new `alpine-324` service
- `Makefile` `PLATFORMS` and `Justfile` `build-r-alpine-324` target
- `.crow/build.yaml` — alpine-324 matrix entries (arm64 + amd64), newest-first

Existing alpine-322/323 matrix entries are left intact.

Reviewed-on: #5

builder/docker-compose.yml

@@ -96,6 +96,19 @@ services:
     volumes:
       - /tmp/alpine-323:/tmp/output/alpine-323
       - ./build.sh:/build.sh:ro
+  alpine-324:
+    command: ./build.sh
+    environment:
+      - R_VERSION=${R_VERSION}
+      - R_INSTALL_PATH=${R_INSTALL_PATH}
+      - LOCAL_STORE=/tmp/output
+    build:
+      context: .
+      dockerfile: Dockerfile.alpine-324
+    image: r-builds:alpine-324
+    volumes:
+      - /tmp/alpine-324:/tmp/output/alpine-324
+      - ./build.sh:/build.sh:ro
   debian-10:
     command: ./build.sh
     environment: