ci(image): publish amd64 and arm64, each built natively

[sajonaro]

Addresses downstream feedback: the published image is amd64-only, so anyone on Apple Silicon pulling ghcr.io/writ-lang/writ runs it under QEMU.

That doesn't fail — it does something quieter. An interrogator that enumerates a state space is exactly the program that turns emulation into a wait, so "published" and "usable by other people" came apart with no error message anywhere.

The approach

A matrix builds each architecture on a runner of that architecture and merges the results into one manifest list — the option the reviewer recommended, over platforms: linux/amd64,linux/arm64 on a single runner.

The one-liner would compile OCaml for arm64 under QEMU on an amd64 host, and run the Dockerfile's stage-2 smoke checks emulated too. That moves the emulation from the user to CI rather than removing it. ubuntu-24.04-arm is free on public repositories, so the honest version costs nothing.

image.yml → two jobs. The matrix pushes each architecture by digest, under no tag; a second job joins the digests with imagetools create and attaches :VERSION and :latest once. Tagging inside the matrix would be two jobs racing for one tag, and the loser's architecture would be the published image — the same bug, harder to see.

image-check.yml → matrixed, for the reason it exists at all: a check that built only amd64 would pass on the PR and leave arm64 to the release. Cache is scoped per architecture, since one shared scope has the two jobs evict each other every run (an arm64 layer is not a usable hit for an amd64 build).

New check

The publish now asserts the tag names both platforms:

for want in linux/amd64 linux/arm64; do
  docker buildx imagetools inspect "$image" | grep -q "$want" || exit 1
done

Nothing else can catch a half-published tag: an amd64-only image pulls and runs perfectly on the amd64 runner that would otherwise be "testing" it. That is precisely how this gap survived the first publish.

Verification

Confirmed by querying the registries directly:

fact	evidence
ocaml/opam:debian-12-ocaml-5.2 has arm64	manifest lists linux/arm64 (also amd64, ppc64le)
debian:12-slim has arm64	manifest lists linux/arm64 v8
ubuntu-24.04-arm is a real label, free on public repos	GitHub Actions runner docs

The arm64 build itself is not verified locally — this machine has no buildx and no binfmt, and docker run --platform linux/arm64 dies with exec format error, so arm64 cannot run here at all. The image-check (arm64) job on this PR is that verification, on real hardware rather than emulation.

Not addressed, deliberately

The reviewer's second paragraph — :latest published alongside 0.1.0, and the client repo pinning the version because make formal's recorded output is a claim about a specific writ — reads as confirmation rather than a request. Both tags are still published; no change.

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