Add glossary, ADRs and future ideas from design grilling

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

docs/adr/0002-library-ui-and-player.md

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+# ADR 0002 — Exercise library, mode design, Player cues
+
+Status: Accepted (2026-10-28)
+
+## Decisions
+
+1. **Library source**: free-exercise-db (Unlicense, 876 exercises). `stretching` → Stretching mode,
+   everything else → Workout mode. No user-created exercises.
+2. **Images**: Bundle both images per exercise, compressed to WebP (~400px, est. 15–20 MB).
+   The start/end "flip" is the v1 animation; real animations are a future step.
+3. **Language**: English only; UI strings centralised for later translation.
+4. **Plan Steps**: Workout = sets, reps-or-duration, optional weight; no rest countdown.
+   Stretching = hold, both-sides flag, rounds. Workout Steps can be grouped into Supersets.
+5. **Library browsing**: search + muscle/equipment/level filters, reused as the Plan exercise picker.
+6. **Structure & design**: Mode switch at top; per-mode tabs Plans · Library · History.
+   Stretching = calm/light (sage/teal, rounded, slow). Workout = dark/energetic (orange/red, bold, snappy).
+   Last-used mode is remembered.
+7. **Player cues**: Visual cues (countdown ring, accent pulse, next cards, auto-advance, screen kept on)
+   plus a gentle, non-annoying sound. No vibration.
+8. **History**: Separate per Mode. The Streak is one shared, Duolingo-style daily Streak across both Modes;
+   only complete Sessions count.
+9. **Units**: kg only.