Add library exercises mid-workout with plan defaults

Pick any exercise from the full library while a workout is running, not
just the ones in its split. The new exercise's plan is seeded from the
most recent log of that exercise, else the library's authored Defaults
line, else a plain 3x10. Adds a searchable two-section picker sheet and
a **Defaults:** bullet to all 47 library reference pages.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Lunge/info.md

@@ -7,6 +7,7 @@ back up through the front heel. Bodyweight first, dumbbells in hand later.
 - **Type:** Split-stance squat (bodyweight or dumbbells)
 - **Targets:** Quadriceps, glutes, balance
 - **Prescription:** 3 × 8–10 per side
+- **Defaults:** 3 × 8 bodyweight
 
 ## Setup