Add iOS platform detection in CMake

[danielraffel]

Series overview

This is the first in a series of 7 small, incremental PRs that add iOS
support to Visage, including multi-touch. Each builds on the previous one:

1. CMake iOS platform detection (this PR) — 2 lines
2. Shared Apple code guards — ~18 lines
3. iOS CMake source routing — ~10 lines
4. iOS windowing implementation + tests — ~570 lines (all new files + Catch2 tests)
5. pointer_id API — ~126 lines (adds int pointer_id = 0 to MouseEvent/Window)
6. Multi-pointer dispatch — ~300 lines (per-pointer tracking in WindowEventHandler)
7. iOS multi-touch wiring + tests — ~385 lines (connect touch IDs to pointer_id + 6 Catch2 test cases)

All changes are backward compatible — desktop platforms are completely
unaffected. These PRs are on my fork for now; happy to adjust the structure
or ordering before submitting upstream.

Summary

Add VISAGE_IOS compile definition when building for iOS, detected before the
existing APPLE check so iOS is distinguished from macOS.

• Adds VISAGE_IOS=1 when CMAKE_SYSTEM_NAME is iOS
• Foundation for subsequent iOS support PRs

Test plan

• Build for macOS: VISAGE_IOS not defined, VISAGE_MAC still set
• Build for iOS: VISAGE_IOS=1 defined, VISAGE_MAC not set
• All existing platforms unaffected