fix(ios): propagate simulator command failures

[justrach]

Summary

• check every simctl subprocess termination status instead of silently discarding it
• surface simctl stderr and return a nonzero status for failed screenshots
• explain that physical-device screenshots require XCUITest and remain unsupported
• clarify the screenshot help text is Simulator-only

Root cause

kuri-mobile captured the raw simctl termination status but all simulator operations discarded it. Passing a physical-device UDID to the default simulator path therefore returned 0 even though simctl rejected the device and wrote no file.

Validation

• Zig 0.16.0: zig build
• Zig 0.16.0: zig build test --summary all (10/10)
• invalid/physical-style UDID smoke: exit 3, no output file, explicit limitation
• --device smoke: exit 3, no output file, explicit XCUITest limitation

Closes justrach/codegraff#265

[justrach]

CI note: the windows-cross-compile failure is inherited from current main (main run 26505539517 is also red). It occurs in generated QuickJS bindings for kuri-fetch with two unused local constants, before the iOS mobile code is compiled, and is unrelated to this two-file screenshot-status patch. The native build-and-test and startup-smoke jobs pass; locally, kuri-mobile builds with pinned Zig 0.16.0, passes 10/10 tests, and both physical-style CLI reproductions exit 3 without creating a file.