Explore Android XR (AI-glasses) integration

[code418]

Summary

Exploratory scaffolding for a future Android XR / AI-glasses surface
(https://developer.android.com/develop/xr/jetpack-xr-sdk/ai-glasses/build).
No behaviour ships. This PR just reserves the build harness and
records a design so the next PR can drop in a real Activity without
reshaping Gradle first.

• New xr product flavour alongside the existing phone and wear
  flavours (minSdk 34, versionCode band +20000, versionNameSuffix "-xr").
• New android/app/src/xr/AndroidManifest.xml — manifest-only source
  set, no activities, no services.
• New docs/smart_glasses_integration.md — feasibility note proposing
  the Android Auto pattern as the closest template: native Kotlin +
  Compose Glimmer, direct Firestore / Functions reads, no Flutter
  engine on glasses (Jetpack XR is Kotlin/Compose only).

What's deliberately not in this PR

• No Kotlin source under src/xr/. The xrImplementation dependency
  block in android/app/build.gradle is committed commented-out
  because the androidx.xr.* artefacts are still alpha and Google
  hasn't pinned coordinates yet.
• No changes to lib/, functions/, firestore.rules,
  storage.rules, pubspec.yaml, firebase.json, the wear source
  set, or the phone source set.
• No CI workflow changes.

Why the Android Auto pattern

The repo already has two non-phone surfaces:

Surface	Engine	Architecture
Wear OS	Flutter (lib/main_wear.dart, wear flavour)	Shares Dart business logic
Android Auto	Native Kotlin (src/phone/.../car/, scoped to phone)	Direct Firestore / Functions reads, no Flutter

Jetpack XR's AI-glasses SDK is Kotlin + Compose Glimmer only, so Wear's
Flutter-entrypoint approach doesn't apply. Android Auto's
StatsRepository.kt + ClaimAction.kt are the proven template — both
will be lifted almost verbatim in a follow-up PR.

Phased rollout (per the design doc)

1. This PR — flavour + doc.
2. Read-only HUD prototype (stats card, mirrors Android Auto's stat
   panel).
3. Voice-driven claim (TTS + ASR wrapping startScoring).
4. Live fuzzy-compass overlay (port of lib/fuzzy_compass.dart into
   Compose Glimmer, sectors only — same no-pinpointing guarantee as
   the phone).
5. Public alpha behind a Firebase Remote Config flag.

Test plan

This PR adds no executable behaviour, so the verification is "we
haven't broken the phone or wear builds":

• flutter analyze still reports no issues (Dart is untouched, so
  this should be a no-op).
• flutter test still reports 105 Dart tests passing.
• cd functions && npm test still reports 268 TypeScript tests
  passing.
• cd android && ./gradlew tasks lists assembleXrDebug alongside
  assemblePhoneDebug and assembleWearDebug.
• ./gradlew :app:processXrDebugManifest merges cleanly.
• ./gradlew :app:assemblePhoneDebug --dry-run succeeds (no
  regression in the default-target build).

Opening as draft because this is future-looking exploration, not a
mergeable feature.

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