darwin: add stable Xcode and iOS simulator shell

[moons-14]

Summary

• add applications.xcode and install stable Xcode from the Mac App Store on Darwin
• include Xcode in workload.development, so the existing m2 host selects it without host-local configuration
• add devShells.ios with an ios-simulator-install command
• make the installer select /Applications/Xcode.app, run Xcode first-launch setup, check newer components, and download the latest stable iOS Simulator runtime

Usage

After applying the Darwin configuration and signing in to the Mac App Store:

nix develop .#ios -c ios-simulator-install

This keeps Xcode installation declarative while leaving Apple's mutable simulator/runtime state to xcodebuild.

Validation

• reviewed repository ownership/Registry rules and the application/service review checklist
• matched the existing shells/android.nix dev-shell pattern
• reviewed the complete task diff before opening this PR
• local execution environment does not provide Nix or macOS/Xcode, so nix flake check, Darwin evaluation/build, and the runtime download command could not be executed here

Manual verification on m2

nix eval --json .#darwinConfigurations.m2.config.homebrew.masApps
nix develop .#ios -c ios-simulator-install
xcrun simctl list runtimes
open -a Simulator