feat: add no-empty-screen-component rule (blank mobile preview / orphan return-null root route)

[mrpopo573]

What

Adds a new laint rule, no-empty-screen-component, that flags a default-exported screen/route component whose body renders nothing: return null, return undefined, an empty return;, or an empty fragment (<></>). A concise-body arrow (() => null) and an identifier default export resolving to such a function are covered too.

Why

Recurring class of blank mobile previews at first boot. The generation agent restructures a mobile app's routes into a route group (for example moving the home UI into (tabs)/index.tsx) but leaves the top-level entry src/app/index.tsx as a stub:

export default function Index() {
  return null;
}

Route / resolves to that top-level index.tsx, renders null, and the app boots to a white screen in both the web preview and Expo Go. The user then burns several retry prompts before it self-resolves.

Evidence (CloudWatch worker logs, us-east-2, 7 day window): the co-occurring Metro metro-virtual/unresolved/throw-*.js bundle error fired 497 times across 97 distinct sandboxes, trending up day over day, with a shared virtual-module signature. This rule catches the static, single-file half of that failure (the return null root route) before it ships.

The rule is intentionally single-file decidable (laint rules receive AST plus source, no filename) and scoped to keep false positives near zero: it only fires on a default-exported PascalCase or anonymous component whose entire body is one empty return. Anything with a conditional early return (loading gate, auth guard, +not-found), any real JSX (including childless elements like <View />), or a non-default helper that returns null is left alone.

Scope

• src/rules/no-empty-screen-component.ts: the rule + meta (severity error, platforms expo + web, category React / JSX).
• tests/no-empty-screen-component.test.ts: 14 cases, both fire and no-fire, including the loading-gate and childless-element non-fire guards.
• src/rules/index.ts and README.md: regenerated by npm run sync (registry entry, README rule table row, rule count 55 to 56). Generated files, not hand-edited.

Testing

• npm test: full suite green (476/476) after npm run build.
• npm run lint, npm run format:check, npm run sync:check, npm run knip: clean for the added files (the pre-existing cli-init lint error and cli/init.ts security warnings are untouched and unrelated).
• End to end via the built CLI: the gift-card stub index.tsx reports the violation and exits 1; a real Gift Cards screen is silent and exits 0.

Linked context

Investigated from build ed1cca00-a8c6-48ec-afd6-ed5961cb5e49 (V2 native, from-scratch). Internal notes only; no customer PII or raw log lines.