ci-requirements: record how to format the Rust lane without a local toolchain

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SreJkbxB4gx8pPsu8QbLPi

ci-requirements.md

@@ -131,3 +131,23 @@ backend/frontend push.
   a TLS-backend change as a change to *this lane*, not just a dependency bump.
 - No Postgres lane (unchanged): the desktop app's local store + sync behavior is
   verified on the operator's machine, not in CI.
+
+## Formatting the Rust lane before pushing
+
+`cargo fmt --check` runs in CI and had failed on four consecutive desktop pushes
+by itself, each costing a full cycle to learn a whitespace nit. There is no Rust
+toolchain on the workstation (rule 10), but the CI image is pullable, and running
+a formatter is neither a test run nor a local stack:
+
+```
+docker run --rm --user "$(id -u):$(id -g)" -e CARGO_HOME=/tmp/cargo \
+  -v "$PWD/desktop/src-tauri:/w" -w /w \
+  git.fabledsword.com/bvandeusen/ci-tauri:1.97 cargo fmt --check
+```
+
+Drop `--check` to apply. `--user` keeps the container from leaving root-owned
+files behind; `CARGO_HOME` points somewhere writable for that user.
+
+**Don't infer formatting from existing code.** Several lines in `local/store.rs`
+exceed 100 characters and survive only because rustfmt cannot break a string
+literal — copying that shape caused one of the four failures.