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.

docs/ci/adr/20260612-adr-ci-002.md

@@ -133,3 +133,13 @@ declaration of the dev *and* build environment.
   flake for `requirements.md` **TT5** (CI runs the tiers) and the
   **D1/D2/D3** distribution items (the release uses a static musl target
   built through this flake).
+
+## Amendment 1 — 2026-06-17: `fmt` gate enabled (issue #35)
+
+The deferred "revisit on `main`" is done. With the CI + website branches
+merged and before the first public release, the tree was reformatted once
+with **stock `cargo fmt`** (no `rustfmt.toml` — stable rustfmt supports no
+meaningful customisation, and the pinned 1.95.0 toolchain makes
+`fmt --check` deterministic) in a single mechanical commit (`41b7e9a`,
+102 files, behaviour-preserving; recorded in `.git-blame-ignore-revs`).
+`ci.yaml`'s gate is now **`fmt --check` + clippy + test**. Closes **#35**.