chore(demo): harden iOS demo run script with preflight checks and simulator auto-boot

[nan-li]

Summary

Improves examples/demo/run-ios.sh based on issues hit when running the Unity demo on iOS from a fresh machine for the first time. Each failure mode previously surfaced as a cryptic, hard-to-diagnose error; the script now fails fast with actionable guidance, or recovers automatically.

• Rosetta 2 preflight (Apple Silicon): Unity's iOS build tooling requires Rosetta 2, but without it Unity batchmode dies instantly with an unrelated dialog. The script now checks for it up front and prints the exact softwareupdate --install-rosetta remediation.
• iOS Build Support preflight: when the Unity editor lacks the iOS module, the build fails with build target was unsupported buried deep in the log. The script now verifies PlaybackEngines/iOSSupport exists for the resolved editor and tells the user to install it via Unity Hub.
• Simulator auto-boot: instead of exiting with No booted simulators found when nothing is booted, the script now lists available iPhones and boots the newest one (opening Simulator and waiting for boot), preserving the existing single-/multi-booted behavior.

Test plan

• On Apple Silicon without Rosetta 2: script exits early with the install instruction.
• With an editor missing iOS Build Support: script exits early naming the module and version.
• With no simulator booted: script lists available iPhones, boots the newest, and proceeds.
• With a simulator already booted: behavior unchanged (uses it / prompts on multiple).
• Full happy path: ./run-ios.sh builds, installs, and launches the demo on the simulator.

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