river-raid: Routine at 68E9

68E9: Initialize bridge state for current player

Clears the top attribute row, removes any active tank shell, and calculates the starting bridge index based on the current player's saved progress. Bridge progression algorithm: The game has 48 unique bridge sections (1-48). Each bridge has 64 terrain fragments. When a player's progress exceeds 48, the algorithm calculates a wraparound: new_bridge = ((progress - 48) mod 15) + 33. This creates an infinite loop through bridges 33-48 (15 bridges) after completing all 48.
init_current_bridge	68E9	LD HL,$5800	Initialize to clear top attribute row (32 bytes).
	68EC	LD B,$20	Initialize to clear top attribute row (32 bytes).
clear_top_attributes_loop	68EE	LD (HL),$00	Clear byte, advance pointer, loop 32 times.
	68F0	INC HL
	68F1	DJNZ clear_top_attributes_loop
	68F3	CALL remove_tank_shell	Clear tank shell and helicopter missile state.
	68F6	LD ($5F73),HL	Clear tank shell and helicopter missile state.
	68F9	LD A,($5F6A)	Load current player's bridge progress (swaps for player 2).
	68FC	LD B,A
	68FD	LD A,($923D)
	6900	CP $02
	6902	CALL Z,get_player_2_bridge
	6905	LD A,B	Check if progress exceeds initial section threshold (48).
	6906	DEC A
	6907	LD DE,$0030
	690A	LD H,$00
	690C	LD L,A
	690D	OR A
	690E	SBC HL,DE
	6910	JP M,finalize_bridge_index
	6913	LD E,$0F	Load section size (15) for division.
divide_progress_loop	6915	OR A	Divide excess progress by 15 to find wraparound offset.
	6916	SBC HL,DE
	6918	JP P,divide_progress_loop
	691B	ADD HL,DE
	691C	LD E,$21
finalize_bridge_index	691E	ADD HL,DE	Calculate final bridge index and fill river attributes.
	691F	LD A,L
	6920	INC A
	6921	LD ($5EF0),A
	6924	JP fill_river_attributes