Add glossary, ADRs and future ideas from design grilling

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

GLOSSARY.md

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+# ZenXercise
+
+An offline Android app for stretching and working out, split into two fully separate Modes. The user
+builds their own Plans from a large built-in Exercise Library and performs them in a guided Player.
+
+## Modes
+
+**Mode**:
+One of the two top-level app states, **Stretching** or **Workout**. Switching Mode changes the whole
+app: design, Exercise Library, Plans and History. The last-used Mode is remembered.
+_Avoid_: Tab, section, category
+
+**Stretching Mode**:
+The calm Mode: light design, sage/teal, rounded shapes, slow transitions.
+
+**Workout Mode**:
+The energetic Mode: dark design, orange/red on near-black, bold type, snappy transitions.
+
+## Exercises
+
+**Exercise**:
+A single movement from the Exercise Library, with name, Instructions, primary and secondary muscles,
+equipment, level and a start and end image. Belongs to exactly one Mode. Users cannot create their own.
+_Avoid_: Move, drill, activity
+
+**Exercise Library**:
+The read-only catalogue of Exercises bundled with the app, sourced from free-exercise-db (876
+Exercises). Category `stretching` belongs to Stretching Mode (123); every other category to Workout
+Mode (753). Browsed with text search and filters for muscle, equipment and level.
+_Avoid_: Catalogue, database
+
+**Flip Animation**:
+Alternating an Exercise's start and end image about once a second, the v1 stand-in for real animation.
+
+## Plans
+
+**Plan**:
+A user-created, ordered list of Steps (and Supersets) belonging to one Mode. The app ships no Plans.
+_Avoid_: Routine, program, workout
+
+**Step**:
+One Exercise inside a Plan, with Mode-specific parameters:
+- *Workout Step*: number of sets; reps **or** duration; optional target weight (kg).
+- *Stretching Step*: hold duration; both-sides flag (left, then right); rounds.
+_Avoid_: Item, entry
+
+**Superset**:
+Two or more Workout Steps grouped so they are performed alternately, one set of each in turn.
+_Avoid_: Circuit, combo
+
+## Sessions
+
+**Session**:
+One run-through of a Plan in the Player. It stores a snapshot of the Plan, so later edits to the Plan
+never change it. A Session is **complete** or **incomplete** (ended early).
+_Avoid_: Workout, run
+
+**Player**:
+The guided, one-Exercise-at-a-time screen used to perform a Session. It shows a large countdown ring,
+pulses the Mode's accent colour in the last three seconds, shows full-screen "next" cards, advances
+automatically when a timer ends, keeps the screen awake, and plays a gentle sound. No vibration.
+
+**Set Log**:
+The reps and weight actually done for one Workout set, prefilled from the Step's targets and confirmed
+with "Set done".
+
+**History**:
+The per-Mode list of Sessions with date, Plan, duration and Set Logs.
+
+**Streak**:
+A single, Duolingo-style daily streak shared by both Modes: consecutive days with at least one complete
+Session in either Mode. Incomplete Sessions do not count.