v0.7.0: Previous passages and dynamic navigation

Added

• @prev Target - Navigate back to the previous passage
  • Use -> @prev in jumps or + [Go back] -> @prev in choices
  • Tracks the immediately previous passage (not decision points like undo)
  • Perfect for menus, inventory screens, side conversations, shop interfaces
  • Works with automatic jumps: in chain A -> B -> C, @prev from C goes to B
  • Persists through save/load (previous_passage_id in save data)
  • Included in GameSnapshot for correct undo/redo behavior
  • Clear error message if used at story start (no previous passage exists)
  • @prev added to reserved targets alongside @join
  • pytest tests: tests/test_prev_target.py (11 tests)