Add glossary, ADRs and future ideas from design grilling

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

docs/adr/0001-v1-foundations.md

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+# ADR 0001 — v1 foundations
+
+Status: Accepted (2026-10-28)
+
+## Decisions
+
+1. **Platform**: Flutter, Android only.
+2. **Modes**: A global Stretching / Workout switch that changes design, content and everything else.
+3. **Content**: A large bundled Exercise Library; users create all Plans themselves (no built-in Plans).
+4. **Data**: Offline only, local on-device storage, no accounts.
+5. **Sessions**: Guided Player (timers, rest countdown, cues) rather than a checklist.
+6. **History**: Simple log (date, Plan, duration) + Streak.
+7. **Visuals**: Simple animations are desired, but may be deferred past v1.
+8. **Scheduling / reminders**: Not in v1.
+
+## Consequences
+
+- No backend; all persistence is a local database on the device.
+- The Exercise Library must be bundled in the APK, so its size (incl. images) matters.