Stabilize iPad landscape CI

[infinityf4p]

Summary

• rotate the iPad simulator before launching the fixture app
• validate the settled landscape frame after the first thread row is available
• retry the isolated iPad UI test at most once for runner-level failures

Verification

• iOS 26.5 iPad Pro 11-inch (M5): CI-equivalent run passed without retry
• same target repeated 3 times with retries disabled: 3/3 passed
• workflow YAML parse passed
• git diff --check passed

This addresses the post-merge main failure in run 32680331379 without weakening the split-layout assertions.