Keep the files many PRs add to conflict-free (#1564)

Message catalogs sorted one message per line (cargo xtask lists [--fix], checked in cargo xtask ci and the Lists workflow); the language list one per line, English first then by id; THIRD-PARTY-LICENSES.txt and docs/features.md rewritten on main by the Generated files workflow instead of in PRs.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

CONTRIBUTING.md

@@ -98,8 +98,8 @@ The [README](README.md#architecture) has the layer table. The short version:
    MIT OR Apache-2.0, and its solver and blends are written from the maths. (Running an
    open-source application to get reference values for a test is fine; reading its source is
    not.) Permissive
-   dependencies are fine. Note which one you're adding and why in the PR, and run
-   `cargo xtask licences` so THIRD-PARTY-LICENSES.txt lists it (CI checks that it is current).
+   dependencies are fine. Note which one you're adding and why in the PR. THIRD-PARTY-LICENSES.txt
+   lists it once the change lands: the Generated files workflow rewrites it on `main`.
    Nightly/manual CI also scans advisories, bans and sources with cargo-deny; see
    [the local command and coverage limits](docs/book/src/dev/testing.md#dependency-advisories).
 3. **Every asset is original or openly licensed and has a row in
@@ -224,6 +224,24 @@ a conflict. `crates/doc/src/format/pending/README.md` has an example.
 files into `crates/doc/src/format/v<N>/` and fixes that number for good (see
 [docs/releasing.md](docs/releasing.md)). Never add a file to a `v<N>/` folder afterwards.
 
+## Lists many changes add to
+
+Some files gain an item in many pull requests at once. They are kept so that two changes that
+each add an item only meet when their items are neighbours, which spares everyone a conflict
+and a fresh CI run:
+
+- **Message catalogs** (`assets/locales/*.json`): one message per line, sorted by id. Add your
+  message anywhere and run `cargo xtask lists --fix`; `cargo xtask lists` (in `cargo xtask ci`
+  and the Lists workflow) fails on an unsorted catalog or an id that appears twice.
+- **`mod` lines**: one per line, in the sorted group they belong to (`cargo fmt` keeps each group
+  sorted). Add a new module into the existing group, not as a new group at the end.
+- **Languages** (`crates/ui-egui/src/i18n.rs`): one per line, English first, then by id (a test
+  checks it).
+- **Generated files**: `THIRD-PARTY-LICENSES.txt` and `docs/features.md` are rewritten on `main`
+  after each merge by the Generated files workflow (`cargo xtask licences`, `cargo xtask
+  features`). Don't regenerate or edit them in a pull request.
+- **Design format**: one file per change, never an edit to a shared number (above).
+
 ## Sending a change
 
 1. Work is tracked in [GitHub issues](https://github.com/bherbruck/solidcraft/issues), labelled