test(i18n): guard against locale keys English has dropped

[joelbrilliant]

The gap

defineLocale() deep-merges a locale's overrides onto en and makes every key optional, so a partially translated locale falls back to English. That's deliberate and good.

What it hides is the opposite case: a locale key English no longer has. The stale override survives the merge and wins over the English value.

TypeScript can't catch it. Blocks like composer.commandDescs are typed Record<string, string>, so any key satisfies the type. Verified on this repo — re-adding a removed key to a locale leaves tsc --noEmit exiting 0.

Why it's worth a guard

This isn't hypothetical. #66646 removed terminal-only commands (/clear, /details, /copy, /quit) from the desktop quick-help drawer because the desktop refuses them. A locale that still defined those keys kept advertising commands that don't work — the exact bug that change fixed, staying live in one language while every other locale was corrected. Nothing failed.

Any long-lived branch touching i18n has this landmine, because a locale added after the branch was cut inherits English silently.

What it checks

Fails when a locale defines a key English doesn't have. Missing keys are not flagged — falling back is the design.

Two exemptions, both documented in the file, because the guard genuinely can't protect them:

• Blocks English leaves deliberately empty — messaging.platformIntro is {}, an extension point locales populate.
• Runtime contribution registries — sidebar.nav is SIDEBAR_NAV_AREA in app/routes.ts; plugins register the ids, so extra locale labels there aren't evidence of a removal.

I kept that list to two and named the reason for each, since every entry is somewhere the guard stops working.

What it found

Running it on current main surfaced two genuinely dead keys in ar.ts, both removed here after checking the app for any remaining reference:

key	why it's dead
keybinds.actions → 'view.closePreviewTab'	no such action exists anywhere in the app
onboarding.flowSubtitles → 'loopback'	desktop handles pkce, device_code, external only

Both would render to Arabic users as descriptions of things that don't exist.

Tests

The walker itself is unit-tested alongside the parity check: reports extra keys, reports nested keys with a dotted trail, does not report missing keys, skips empty blocks, skips registry paths, and ignores functions/arrays.

39 passed in src/i18n, tsc clean, lint clean.

[teknium1]

Thanks for adding a behavioral invariant rather than a locale snapshot. Current main still has both stale Arabic overrides: view.closePreviewTab at apps/desktop/src/i18n/ar.ts:236 and loopback at apps/desktop/src/i18n/ar.ts:1963; English has no corresponding keys (apps/desktop/src/i18n/en.ts:2313-2317). defineLocale() retains arbitrary override keys during its merge (apps/desktop/src/i18n/define-locale.ts:25-40), so the guard addresses a real current defect. The PR is currently CLEAN and MERGEABLE according to gh pr view 71233.

Automated hermes-sweeper review.

[joelbrilliant]

Thanks for checking the stale-key premise and the behavioural guard against current main. The current head is 316007f63, the PR is clean and mergeable, and all required checks are green.