Fix CI: add explicit xcodebuild destinations and update Xcode matrix (#1594)

- Add explicit -destination flags to build.sh — newer Xcode versions no
  longer auto-select a build destination, causing "Found no destinations
  for the scheme" errors
- Update CI matrix to Xcode 16.4 and 26.2 — Xcode 16.0-16.3 simulator
  runtimes are no longer installed on the macos-15 runner image
- Add PRODUCT_BUNDLE_IDENTIFIER to ExternalTarget fixture — Xcode 26.x
  requires a bundle identifier for embedded frameworks

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>