some more fixes

simplify/guidelines/code-style.md

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+# Code style
+
+## Formatting
+
+No prettier. No formatter of any kind. The source is hand-formatted at roughly 120 columns and there
+is no config to make a formatter agree with it.
+
+Running `npx prettier --write` rewrites a file at prettier's 80-column default and turns a 60-line
+change into a 340-line diff, which destroys reviewability and churns files the change never touched.
+If one has already run, `git checkout <file>` and redo the edits by hand.
+
+Match the surrounding indentation. Make surgical edits.
+
+There is no eslint either. The gate is `tsc`, `cargo check` and the test suites.
+
+## Comments
+
+The rule as originally written was "no code comments: make the code readable instead". That is still
+right about one kind of comment and wrong about another, and the codebase has moved.
+
+A comment never says what the code does. If you are about to write one, rename something or extract
+a function instead.
+
+A comment does say why a decision was made, when the reason is not recoverable from the code. The
+best examples in the suite are `shared/src/fonts.ts`, `shared/src/icons.ts` and the dependency
+blocks in Margin Mail's and Margin Docs' `Cargo.toml`, which explain why a version is pinned exactly,
+why `keyring` is not used, why `bundled` is the feature that gets FTS5. Every one of those is a
+decision somebody would otherwise undo by accident.
+
+The test: delete the comment and ask whether a competent person would make the same mistake twice.
+If yes, keep it. If it just narrates the line below, cut it.
+
+## Tests
+
+The old rule was "never commit tests, verify by running the real product". That rule is dead. It was
+true of Margin alone and Margin has no test script to this day, but Margin Calendar, Margin Docs and
+Margin Mail all ship vitest unit tests and Playwright suites, and they catch things.
+
+The position now: unit tests go beside the module they test as `Name.test.ts`, and they test pure
+model code (`GridModel`, `AgendaModel`, `QuickCreateModel`, `theme`, `width`, `providers`), never
+rendering. Playwright suites drive the app in a browser against the `src/dev` fixtures.
+
+Running the real product is still required, and still the last step. Tests are in addition to it,
+not instead of it. See [working-together.md](working-together.md).
+
+If the user wants the old rule back for a given app, they will say so. Do not delete an existing
+suite on the strength of a memory written before it existed.
+
+## Cross-platform over per-OS native
+
+Before adding a dependency with per-OS backends, check it covers all five targets: macOS, Linux,
+Windows, Android, iOS. If it does not, prefer one implementation that works everywhere, and state
+the security or capability trade plainly in the code rather than hiding it behind a fallback chain.
+
+`keyring` is the worked example and the reason for the rule. It had four ways of reaching one real
+implementation. macOS was already excluded because the Keychain ties an item to the code signature
+and re-prompts on every rebuild; Android has no backend at all; on Linux the Secret Service is
+missing on exactly the minimal window managers that most wanted it. It was replaced everywhere by
+XChaCha20-Poly1305 sealed files in the app data directory. Accepting a weaker but uniform mechanism
+is usually the right answer here.