Android: extend CI to build the Android example

[LeslieOA]

Last code-side slice of #32 (the docs sub-task is then all that remains).

Scope

• Add a `build-android` job to `.github/workflows/ci.yml` mirroring the existing `build-ios` job. JDK 17, Android SDK setup, `expo prebuild --platform android`, then `./gradlew :app:assembleDebug`.
• Fix existing iOS job paths that still reference the old `example/ios-app/` directory + `SourceEditorExampleiOS.xcworkspace` scheme. After PR feat(android): Fabric library scaffold + MVP (closes #33, refs #32) #34 (already merged) the directory is `example/expo-app/` and a fresh prebuild generates `SourceEditorExample.xcworkspace` (slug change). The CI job hasn't run since these renames so the breakage is latent.
• Add a `build-macos` job while we're in there — the comment block at the bottom of ci.yml flagged it as pending macOS-app, which has shipped (feat(macos): macOS support via RN's first-party SPM helper #30). Mirrors the macOS dev workflow: `pod install` + `xcodebuild` against `MacosApp.xcworkspace`.

Out of scope

Linting (no eslint config in the repo), unit tests (none yet), distribution builds.

Refs #32.