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

.crow/build.yaml

@@ -56,6 +56,14 @@ matrix:
     #   ARCH: arm64
     #   ARCH_ID: aarch64
     #   INSTANCE_TYPE: cax31
+    - PLATFORM: alpine-324
+      PLATFORM_ID: alpine324
+      ARCH: arm64
+      ARCH_ID: aarch64
+    - PLATFORM: alpine-324
+      PLATFORM_ID: alpine324
+      ARCH: amd64
+      ARCH_ID: x86_64
     - PLATFORM: alpine-323
       PLATFORM_ID: alpine323
       ARCH: arm64