[Test] shellframe_app has zero test coverage

[fissible]

Problem

`shellframe_app` (the multi-screen driving loop in `src/app.sh`) has no unit tests and no integration tests. It is featured in `docs/showcase.md` and is the primary API for multi-screen flows, but its behavior is entirely unverified.

What needs coverage

• Screen type dispatch: `confirm`, `alert` types correctly routed
• `_SHELLFRAME_APP_NEXT` transitions: render → widget → event → next screen
• `QUIT` sentinel exits the loop
• Details array (`_SHELLFRAME_APP_DETAILS`) forwarded to widget correctly
• Event handler naming convention (`PREFIX_SCREEN_()`)
• Guard against infinite loop when event handler does not set `_SHELLFRAME_APP_NEXT`

Suggested approach

Integration tests via ptyunit — send key sequences through a two-screen app and assert the screen transitions and exit code.

Effort

S (1–2h)

Depends on

None (can be written against the current API)