ci: add the publish.yaml workflow file (completes d3af1c4)

d3af1c4 described the manual publish workflow and updated ADR-0056, but
`git commit -am` doesn't stage new untracked files, so publish.yaml
itself was left out. Add it here.

.gitea/workflows/publish.yaml

@@ -0,0 +1,79 @@
+# Manual publication workflow (workflow_dispatch) — the outward, irreversible
+# release steps a human triggers AFTER the automated `release.yaml` build has
+# produced downloadable assets (and they've been eyeballed as good).
+#
+# Why manual + separate from release.yaml:
+#   * Publishing to a public registry is irreversible (crates.io versions can
+#     only be *yanked*, never deleted) — a human pulls this lever, and the
+#     registry token never sits on every tag push.
+#   * Our release is split (Linux/Windows on the tag, macOS dispatched), so a
+#     human is the natural "all assets are up — go" gate. crates.io publish
+#     reads SOURCE so it doesn't strictly need the release, but binstall's
+#     metadata points at the release assets — hence run this once builds exist.
+#
+# Structure: each registry is its OWN job with NO inter-job `needs`, so jobs run
+# independently and one failing (or a newly-added one) never breaks another.
+# Every job is IDEMPOTENT — re-dispatching when a target is already published is
+# a clean no-op. Add Scoop / Homebrew / winget as sibling jobs here later.
+name: publish
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Release tag to publish (e.g. v0.2.0)'
+        required: true
+
+jobs:
+  crates-io:
+    runs-on: ci-public
+    container:
+      image: git.lazyeval.net/oli/rdbms-playground-ci:latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ inputs.tag }}
+
+      - name: publish to crates.io (idempotent)
+        shell: bash
+        env:
+          TAG: ${{ inputs.tag }}
+          # A crate-scoped, publish-update crates.io token, stored as a Gitea
+          # Actions secret. `cargo publish` reads CARGO_REGISTRY_TOKEN from env.
+          CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}
+        run: |
+          set -euo pipefail
+
+          # Source of truth = the [package] version at the checked-out tag
+          # (toolchain-free read; same approach as release.yaml's guard, which
+          # avoids the flake devShell's stdout banner corrupting a parse).
+          VER=$(grep -m1 '^version = ' Cargo.toml | sed -E 's/^version = "(.*)"/\1/')
+          [ -n "$VER" ] || { echo "ERROR: could not read version from Cargo.toml" >&2; exit 1; }
+          if [ "$TAG" != "v$VER" ]; then
+            echo "ERROR: dispatch tag '$TAG' != 'v$VER' (Cargo.toml at that tag)" >&2
+            exit 1
+          fi
+
+          # Idempotency: if this version is already on crates.io, no-op.
+          # (crates.io requires a descriptive User-Agent per its data policy;
+          # without one the API returns 403.) Only an explicit 200 means
+          # "already there" — anything else proceeds, and `cargo publish` is the
+          # final backstop (it refuses to overwrite an existing version).
+          UA="rdbms-playground-release-ci (oliver@sturmnet.org)"
+          code=$(curl -sS -o /dev/null -w '%{http_code}' -A "$UA" \
+            "https://crates.io/api/v1/crates/rdbms-playground/$VER" || echo 000)
+          if [ "$code" = "200" ]; then
+            echo "rdbms-playground $VER is already on crates.io — nothing to do."
+            exit 0
+          fi
+          echo "crates.io returned HTTP $code for $VER (not 200) — proceeding to publish."
+
+          echo "publishing rdbms-playground $VER to crates.io ..."
+          nix develop -c cargo publish --locked
+          echo "published rdbms-playground $VER to crates.io."
+
+  # Future manual publication targets go here as independent, idempotent jobs,
+  # e.g.:
+  #   scoop-bucket:   { ... update the lazyeval Scoop bucket manifest ... }
+  #   homebrew-tap:   { ... update the lazyeval Homebrew formula ... }
+  #   winget:         { ... komac submit, or a manual PR helper ... }
+  # No `needs:` between them — each checks the target and skips if already done.