Fix non-root initial route Back history

[artiphishle]

Fixes #158

Summary

• generate non-root auth root redirects with Expo Router's withAnchor
• keep the redirected destination as the initial Stack history entry instead of leaving / behind it
• retain the canonical root redirect and auth bootstrap/protected-route boundary introduced by Fix generated auth root reload and bootstrap boundary #155
• extend focused generation coverage for the root, app, and tabs initialRouteName chain
• add a patch changeset

Root cause

For authenticated apps whose configured initial/post-sign-in route is non-root, the generated src/app/index.tsx redirected to /products without re-anchoring navigation. Entering through / could therefore leave an earlier Stack entry behind the Products screen and expose an erroneous Back button.

Expo Router supports withAnchor on Redirect specifically to replace the initial screen with the redirected route. The generated route now uses:

<Redirect href={'/products'} withAnchor />

Impact

• opening / resolves to /products without an erroneous Back button
• opening /products directly remains unchanged
• nested routes retain normal Back navigation to their configured initial screen
• no header is manually hidden; navigator history remains authoritative

Validation

Added focused Bun generation assertions covering:

• anchored /products redirect generation
• root Stack initial route (app)
• authenticated app Stack initial route (tabs)
• tabs initial route products
• generated Products route placement

Full repository checks are left to CI because this change was authored through the connected GitHub environment rather than a local checkout.