ci: enable the cargo fmt --check gate (ADR-ci-002 Amendment 1)

Adds `cargo fmt --check` (stock defaults) to ci.yaml's gate, now that the
tree is rustfmt-clean (commit 41b7e9a). Records that reformat in
.git-blame-ignore-revs so `git blame` skips it. Amends ADR-ci-002 (the
deferred "revisit on main" fmt decision) + the ci ADR index.

Closes #35.

.gitea/workflows/ci.yaml

@@ -2,9 +2,10 @@
 # build-ci-image.yaml), so the pinned 1.95.0 toolchain is already warm — steps
 # just enter the flake devShell and run cargo.
 #
-# Gate = clippy + test. fmt is deliberately NOT gated yet (ADR-ci-002: the tree
-# isn't clean under stock rustfmt; revisit on main). The release job (static
-# binary for D2) and the platform matrix layer on later, step by step.
+# Gate = fmt + clippy + test. The fmt gate (`cargo fmt --check`, stock defaults)
+# was enabled once the tree was reformatted on main (ADR-ci-002 Amendment 1 /
+# issue #35). The release job (static binary for D2) and the platform matrix
+# layer on later, step by step.
 name: ci
 on:
   push:
@@ -39,6 +40,8 @@ jobs:
       image: git.lazyeval.net/oli/rdbms-playground-ci:latest
     steps:
       - uses: actions/checkout@v4
+      - name: fmt (check, stock defaults)
+        run: nix develop -c cargo fmt --check
       - name: clippy (warnings denied)
         run: nix develop -c cargo clippy --all-targets -- -D warnings
       - name: test