Document iOS build prerequisites in README

[dnorman]

Summary

Building the iOS app on a fresh machine (especially Xcode 26) has several prerequisites that aren't documented and produce confusing failures when missing.

To document

• Node path for Xcode script phases — resolved via ios/.xcode.env (command -v node) with an optional ios/.xcode.env.local override. A stale .xcode.env.local (e.g. pointing at a removed nvm version) makes every RN script phase fail; because the Hermes "Replace Hermes" phase runs first, it looks like a Hermes problem when it's really "node not found". Document how to set it and this failure mode.
• Xcode 26 / iOS 26 SDK — the fmt consteval issue and the Podfile post_install patch that works around it (link the RN-upgrade issue).
• CocoaPods locale — under a non-UTF-8 locale, pod install can crash with Unicode Normalization not appropriate for ASCII-8BIT (Encoding::CompatibilityError). Run with LANG=en_US.UTF-8 LC_ALL=en_US.UTF-8.
• Simulator — dev.sh matches iPhone 16 (; document creating that sim (or relaxing the match to accept e.g. iPhone 16e).
• Maestro e2e — requires Java 17 (JAVA_HOME=/opt/homebrew/opt/openjdk@17) plus the maestro CLI on PATH.

Acceptance

• README has a "Building / prerequisites" section covering the above