Add an experimental Alpine image flow

Stage a complete Alpine x86_64 image stack so \	--image alpineworks like the existing manual Void path instead of relying on Debian-oriented image builds.\n\nAdd make targets plus kernel/rootfs/register helpers that download pinned Alpine artifacts, extract a Firecracker-compatible vmlinux, build a matching mkinitfs initramfs, seed OpenRC services, and register/promote a managed image named alpine.\n\nFold in the bring-up fixes discovered during boot validation: use rootfstype=ext4 in shared boot args, install libgcc/libstdc++ for the opencode binary, and give opencode more time to become ready on cold boots.\n\nValidate with go test ./..., the Alpine helper builds, image promotion, and banger vm create --image alpine --name alp --nat plus guest service and port checks.

README.md

@@ -189,6 +189,46 @@ That flow uses:
 - `./build/manual/rootfs-void.ext4`
 - `./build/manual/rootfs-void.work-seed.ext4`
 
+## Experimental Alpine Flow
+
+Stage an Alpine virt kernel:
+
+```bash
+make alpine-kernel
+```
+
+Build the experimental Alpine rootfs:
+
+```bash
+make rootfs-alpine
+```
+
+Register it:
+
+```bash
+make alpine-register
+```
+
+Create a VM from it:
+
+```bash
+./build/bin/banger vm create --image alpine --name alpine-dev
+```
+
+That flow uses:
+
+- `./build/manual/alpine-kernel/`
+- `./build/manual/rootfs-alpine.ext4`
+- `./build/manual/rootfs-alpine.work-seed.ext4`
+
+The experimental Alpine flow stages a pinned Alpine release by default. Override
+that pin with `ALPINE_RELEASE=...` when running the `make alpine-kernel` and
+`make rootfs-alpine` helpers if you need a different patch release.
+
+Alpine support currently applies to the explicit register-and-run flow above.
+The generic `banger image build --from-image ...` path remains Debian/systemd-
+oriented and should not be treated as an Alpine image builder.
+
 ## Notes
 
 - Firecracker is resolved from `PATH` by default.