maui: phase 6 (Essentials) — docs verdict, no code change needed

[jonathanpeppers]

Verdict: Phase 6 is a no-op

After investigation, no Compose-specific Essentials implementations
are needed.

Microsoft.Maui.* Essentials APIs (IBattery, IConnectivity,
IPreferences, ISecureStorage, IGeolocation, IFilePicker,
IShare, IClipboard, IBrowser, IHapticFeedback, IVibration,
IFlashlight, IEmail, ISms, IPhoneDialer, IMediaPicker,
ILauncher, IFileSystem, IAppInfo, IDeviceInfo) wrap platform
APIs (Intent, SharedPreferences, EncryptedSharedPreferences,
LocationManager, ClipboardManager, Vibrator, Camera2, system
services) and are independent of the handler chain.

UseAndroidXCompose() only swaps handler registrations — it does not
touch the Essentials DI registrations — so MAUI's stock Android
Essentials implementations resolve unchanged.

The sample uses MauiAppCompatActivity (unchanged by this backend),
so even APIs walking Platform.CurrentActivity find the activity
type they expect.

What changed

• docs/maui-backend.md — Phase 6 section rewritten as the
  investigation verdict + per-API coverage table + "when future work
  might be needed" caveat. The only future case is a
  Compose<IFilePicker> / Compose<IShare> overlay if a Material 3
  native picker UX is ever required; the system intent is the
  platform-correct UX, so this stays a deferred follow-up.

No production code or sample changes — by design.

[Copilot]

Pull request overview

This PR closes out Phase 6 (Essentials) of the MAUI Compose backend with a no-op verdict: all Microsoft.Maui.* Essentials APIs work unchanged under UseAndroidXCompose() because they wrap platform APIs (Intent, SharedPreferences, LocationManager, etc.) that are independent of the handler chain. The PR delivers a comprehensive sample page proving this, wiring, manifest permissions, and documentation.

Changes:

• New EssentialsPage (XAML + code-behind) exercising ~18 Essentials APIs with per-row result labels for on-device verification, plus AppShell route and HomePage catalog entry.
• AndroidManifest.xml gains VIBRATE, location, FLASHLIGHT, and CAMERA permissions needed by the interactive Essentials APIs on the page.
• docs/maui-backend.md Phase 6 section rewritten from a speculative plan to a definitive investigation verdict with a per-API coverage table and future-work caveat.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Microsoft.AndroidX.Compose.Maui.Sample/Pages/EssentialsPage.xaml	New XAML page layout with snapshot labels, storage round-trip buttons, feedback/intent/permission-gated sections.
src/Microsoft.AndroidX.Compose.Maui.Sample/Pages/EssentialsPage.xaml.cs	Code-behind populating snapshot labels on load and handling button taps for each Essentials API with result feedback.
src/Microsoft.AndroidX.Compose.Maui.Sample/HomePage.xaml.cs	Adds "Essentials" entry to the gallery index.
src/Microsoft.AndroidX.Compose.Maui.Sample/AppShell.xaml.cs	Registers "essentials" Shell route.
src/Microsoft.AndroidX.Compose.Maui.Sample/Platforms/Android/AndroidManifest.xml	Declares permissions for Vibration, Location, Flashlight, and Camera.
docs/maui-backend.md	Replaces speculative Phase 6 plan with the no-op investigation verdict, API coverage table, and future-work notes.