Stabilize inherited PTY integration tests

[myobie]

What changed

• wait for an explicit marker emitted after alternate-screen exit before asserting that the main buffer was restored
• register both CLI-spawned nesting-test daemons with the existing awaited teardown barrier

Why

PR #128's hosted Vitest gate exposed two inherited test races. The screenshot test waited for text written before alternate-screen entry, so it could snapshot during the deliberate 300 ms alternate-screen interval. The nesting tests signaled two detached daemons directly without awaiting their exit, allowing them to recreate state while the suite removed its temporary root.

Production behavior is unchanged; this PR modifies tests only.

Validation

• alternate-screen focused case: 20/20 repeated passes
• nesting suite: 20/20 repeated passes
• npm run typecheck
• npm run build
• npm run verify-docs (13/13)
• combined screenshot + nesting files: 45/47 locally; only the two known host Nano failures (Incomplete terminfo entry), while both corrected cases pass