Document the Library tab in the changelog and README

Claude-Session: https://claude.ai/code/session_01H8VxUX4ckjU3vRF5M4L5FV

CHANGELOG.md

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 **July 2026**
 
+A new Library tab puts your routines and the exercise library one tap away instead of buried in Settings.
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+Drag routines into your own order — the Today board and routine pickers follow it everywhere.
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+Swipe right on any routine to duplicate it as a fresh copy you can evolve freely.
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+Starter routines now wear a Starter badge, and editing one explains that your changes become your own copy.
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+A starter gallery previews every built-in routine — including ones you've deleted — and re-adds them one at a time or all at once.
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+Each routine now shows when you last trained it, how many workouts you've completed with it, and its scheduled days.
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+Deleting a routine now warns you when scheduled days still point to it.
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+The exercise library and the add-exercise picker are now grouped by category and searchable by name, category, or muscle.
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 Reopening an exercise on the watch now also continues its timer where it left off — even mid-rest — instead of restarting from zero.
 
 A renamed exercise's headline now shows both identities on one line, like "Cardio · Warm Up".