Add native platform CI build jobs (xcodebuild, gradle, cargo)

[enricopiovesan]

Summary

Add CI workflow jobs that compile native traverse-starter platform clients to catch breakage early.

Why

Native clients (iOS, macOS, Android, Windows, Linux GTK, CLI) ship in-repo but CI currently validates only Node/web checks. Platform scaffolds can drift without compile-time feedback.

Depends on

• Shipped platform scaffolds under apps/traverse-starter/

Blocked by

• CI runner availability (macOS for Xcode, Windows for WinUI) and maintainer decision on which platforms are merge-blocking vs advisory

Definition of Done

• .github/workflows/ci.yml (or dedicated workflow) includes build jobs per platform:
  • iOS/macOS: xcodebuild (or swift build where applicable)
  • Android: ./gradlew assembleDebug (or equivalent)
  • Linux GTK + CLI: cargo build
  • Windows WinUI: MSBuild step (may require windows-latest runner)
• Jobs are documented in docs/quality-standards.md
• Failures produce actionable logs; optional platforms may be continue-on-error until stabilized

Validation

• Push a PR that breaks a native compile → relevant job fails
• Green main with all enabled native build jobs passing