Add glossary, ADRs and future ideas from design grilling

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/adr/0003-tech-stack-follows-cookit.md

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+# ADR 0003 — Tech stack follows CookIT
+
+Status: Accepted (2026-10-28)
+
+ZenXercise uses the same stack and conventions as the author's CookIT app (local-first Flutter, Android),
+so both codebases read alike.
+
+- State: `provider` with `ChangeNotifier` controllers, wired in `main.dart` via `MultiProvider`.
+- Persistence: `drift` + `drift_flutter` (SQLite); `shared_preferences` for small settings (last Mode).
+- Navigation: plain `Navigator` with named routes (no go_router).
+- Strings: `flutter gen-l10n` with ARB files in `lib/src/localization` (`app_en.arb` template).
+- Layout: `lib/src/<feature>/` with repositories, controllers and `views/`.
+- Screen-on in the Player: `wakelock_plus`. IDs: `uuid`.
+- Tests: `flutter_test`, `mocktail`, `sqlite3` (in-memory drift in tests).
+- Lints: `flutter_lints`. Docs: `GLOSSARY.md` (term / definition / _Avoid_) and `docs/adr/`.
+
+Supersedes the Riverpod / go_router proposal from the grilling session.